* privacy addresses are great

* deriving additional addresses for specific functions is great (e.g. XLAT464/CLAT)

* you don't get collisions when you lose your DHCP lease database

* as Brian says, DHCP wasn't quite there yet when IPv6 was designed

* ability to proactively change things by sending different RAs (e.g. router or prefix failover, though these don't work as well as one would hope)

* ability to encode mnemonic information into those 64 bits (when configuring addresses statically)

* optimization for the routing layers in assuming prefixes mostly won't be longer than /64

… and probably 20 others that don't come to mind immediately. I didn't even spend seconds thinking about the ones I listed here.

DHCP requires explicit configuration; it needs a range that hopefully doesn't conflict with any VPN you use; it needs changes if your range ever gets too small; and it's just another moving part really.

With SLAAC, it's just another implementation detail of the protocol that you usually don't have to even think about, because it just works. That is a clear benefit to me.