Edit: My question wasn't so much why you'd want SLAAC as why you'd need a daemon for it. From https://www.openbsd.org/papers/florian_slaacd_bsdcan2018.pdf:
> Remove sending of router solicitations and processing of router advertisements from the kernel.
Oh, got it. That totally makes sense in an OpenBSD way.
YMMV!
edit: I may be slightly misunderstanding, and it might not be needed. Regardless, my router can now ping internet addresses while running slaacd.
Perhaps you don’t want it.
I could go off and RTFM, but then we wouldn't get to talk about it.
Hopefully that's clear as mud ;) I would encourage you to go check out IPv6 if that was the intent of your original question. It actually makes more sense after you dive in, and can be pretty neat.
ULAs (Unique Local Address) are one often-overlooked part of which I'm an advocate.
1. It can be one so it probably should be. 2. This lets you not run it.