The big one would be having a phone that's still supported with updates to the OS to patch security exploits. By the time the 11 comes out, I wouldn't be surprised if patches stopped coming to the S8 to force people to buy the newer phones. That's just business, and while I despise that thought, it's still a very real thing.
To others, the faster processor could be essential to their daily lives if they're always busy and need their phone for their work. To me, like most others, I just like having a new thing when my old thing feels old. It's a Feelycraft thing for sure, but it's also mixed with the first point of still having updates. By the time I upgraded my S2 and S4, they were no longer being supported by updates.