Side quests are there for solo exp. They are intended to be completed whilst levelling up, for example while waiting for the DF to pop. This is why they also give levelling gear, materia and Gil as rewards, to help you as you level, which is stuff you don't get from FATES, leves and PotD. They also have little tidbits of lore that help flesh out the world.

If you are at 70 and you are not interested in story then yes, I can see why you don't feel incentivised to do them, but there is no point in saying 'give me a reason do them'- there already is a reason, it just doesn't interest you.