Why do Kaioshins have Potara Earrings with the ability to fuse with one another?

[Asura]

Any good theories on this? What could be the reasoning behind Kaioshin's getting these powerful items that allow them to permanently fuse with someone else? Why do they wear it on their ears too?

[theherodjl]

Survival probably, Kaioshin are stronger than the average lifeforms they create but they aren't invincible. Kaioshin can still be overwhelmed & killed which would in turn also kill the Hakaishin and remove the angel from their position, the ruling hierarchy of the universe being brought to an abrupt close. The Potara were probably created to enhance the strength of a Kaioshin so that the fragile balance of power on their end wouldn't be such a problem.

[Pantalones]

Probably as a defense against Buu, considering that he's apparently ALWAYS existed and the older, more experienced Kaioshin presumably would have known about him.... even though the younger East Kaioshin apparently didn't until Bibidi unleashed him onto the universe. South Kaioshin's strength and Dai Kaioshin's magical weirdness combined with a significant boost of power would almost certainly be capable of handling the original Buu.

Maybe there were (are?) other ancient, super-powerful beings floating around that they needed extra help dealing with from time to time. Dabura was about as strong as Cell (and above the level of the East Kaioshin), so Potara fusion being intended to deal with threats coming from the Demon Realm is another possibility. If you want to count the movies, the list of "more than East Kaioshin (and maybe North/West too) can handle" enemies gets a lot longer. Bojack was also around the level of Cell, and his gang were approximately as strong as Cell Juniors -- so Bojack alone would be overwhelming for all but the strongest Kaioshin, and even his minions would be extremely dangerous to the weaker ones, especially if they ganged up on them. Something like Janemba could've happened at any time if the soul cleansing machine in Hell malfunctioned or was neglected -- so that's a potential Buu-level threat that you'd probably need a South/Dai Kaioshin fusion to handle. Hirudegarn was another Buu-level "disaster waiting to happen" that the Kaioshin could've been aware of -- no surprise that he's in the South Kaioshin's jurisdiction, one of the strongest potential threats being watched over by the strongest creator god.

As for why they didn't fuse when Buu attacked... from what we hear it seems like the North and West died too fast for fusion to even be brought up as a possibility, and then the East was attacked and incapacitated but not killed. Then he said that the South was "somehow" absorbed... does that imply he didn't actually see it happen (knocked out from Buu's attack on him earlier?), or just that he doesn't understand how Buu's absorption works, or what? Hard to say, but it's safe to say that the East was in no shape to fuse with the South at this point. Even if he was, unless the South was powerful enough to actually fight Buu near-evenly like the anime flashbacks say, it's doubtful that the East Kaioshin would've contributed enough power to make a difference, especially 5 million years ago when he was even weaker and even more inexperienced than he is in the present! And of course, by the time Buu went after the Dai Kaioshin there weren't any other Kaioshin left to fuse with, aside from the injured East Kaioshin who definitely wouldn't have been enough to make a fusion capable of handling Buff Buu.

[SupremeKai25]

The Potara earrings also allow the wearer to use the Time Rings. It is why Zamasu had to kill his master and steal his Potara to use the Time Rings and set in motion his Zero Mortals Plan. However, it would seem that the apprentice has weaker Potara earrings than the Supreme Kai (which is logical). So there is a close relationship between the Potara earrings and the ominous Time Rings.

While it isn't stated in the canon material why the Potara have this Fusion capability, I assume it is meant to be a last resort in case everything else is lost against a powerful enemy. Black himself states that the time has come to show his enemy the 'true power of the Gods'. The Potara fusion is not intended to be a weapon you casually unleash at the beginning of the fight, it's a weapon meant to be saved until the very last moment, when all hope of victory seems lost, and it represents the true and purest power of the Gods. The Gods of Creation have proven to be quite cunning and strategic, having devised a perfect weapon to avoid utter defeat. Which makes its use by mere mortal hands even more unforgivable and preposterous. In fact, the time-limit on the Potara for mortals could be an intenional design flaw to avoid mortal beings from exploiting the power of the Gods.

[8bitdee]

Because Toriyama probably thought ear-rings were a good indicator to portray a higher being in the gods hierarchy and then needed a plot device to have Goku and Vegeta fuse without the fusion dance.

There is no explanation to your question because the question assumes that Toriyama modeled Shin and Kibito at the beginning of the Buu arc with the idea that the ear-rings had any sort of narrative importance, when we all know that's not Toriyama wrote DB and the appearance of ear-rings were simply for aesthetic purposes.

[sunsetshimmer]

The REAL answer was already said by 8bitdee.
Toriyama just designed them that way. He needed a way to fuse Goku with Vegeta, but since Gogeta already appeared in a move, Toriyama wanted to make new type of fusion. He then decided to use earrings.

[dbgtFO]

Out of universe, aside, Elder Kaioshin did state it was their secret weapon, so if worst came to worst, they would have that as emergency backup. And of course as of Super, the Potara are linked with the timerings.

[Zillamon51]

Maybe powerful mortals using the fusion dance once rose up against the gods, so the gods used their mystical powers to create a more powerful means of fusion.

[Dorexx]

From a more general perspective I think it's interesting that gods (creation gods especially) would have such an ability, basically to combine/tamper with souls, since their job is to create souls. It feels like something a higher being/god/creator would be capable of doing.

Although it's odd why it's permanent only for them.

[Baggie_Saiyan]

From a more general perspective I think it's interesting that gods (creation gods especially) would have such an ability, basically to combine/tamper with souls, since their job is to create souls. It feels like something a higher being/god/creator would be capable of doing.

Although it's odd why it's permanent only for them.

I guess it makes sense since it is the Kaioshin's secret weapon I am sure they would want it to last a significant time and being permanent would also stop them spamming it and only use it as an emergency.

[Ozotto]

They have them so old ugly witches can feel pretty.

..well at least briefly before becoming an elderly kai.