Besides financial bonus, new stadium upgrades, increased attendance limit (up to level ~16 I think) and, if you reach top 4, playing Champions League, there is no other real reason to get promotion.
Promotion is also useful for new players to reach level 4 and unlock negotiations.
For me, promotion is good only if I finish in top 4, otherwise I don't want promotion at all. Those financial bonuses we get for 5th, 6th and 7th place are nowhere enough for reconstruction of team for next season. My team has world arena built so now I don't care about stadium upgrades