Performance gains for certain software or games. Especially if you are into emulation then the higher power of newer flagships or better cooling design, mean that you can run more recent games on your phone. Same thing goes with camera lenses, better camera means you can get better shots so if you are into photography it makes sense to upgrade.
Thrift wise, there is never really a reason to upgrade as long as it still texts and makes calls but the non-phone features are why you would upgrade early.