1) Toryiama seeing himself as old man so he see Master Roshi as his image. By this Toryiama wants to show young generation that he is still important and relevant, so some young guys and girls would notice it in real life as well that old people are not useless. That why he written Roshi can be as strong as he feel need to be.
2) Fan service, Old man is iconic character of db universe so he was brought back to spotlight due to nostalgia effect of older people while showing new generation greatness and usability of these old characters who were forgotten for long and caster away.
3) Gag character, Roshi is definietly character who can be entertaining and making laugh of his doings or his behaviour, even face expressions
4) Because old man look badass
5) Toryiama is going to pull of the biggest troll move with Roshi ever. Due to not killing rules Roshi will sacrifice himself to eliminate Jiren while he would charge at Goku to ring him out, so Jiren would be eliminated out due to killing Roshi. I believe it would be the greatest troll move and usefull death for old man saving universe.
6) The greatest leader ever to lead other characters using crafty strategies and techniques
Just wondering if it happen would omniking get pissed to see goku let down not being able to face of Jiren so he would unset rules so it would be death battle

