How does SLAAC work, and when do I still need DHCPv6?

SLAAC (stateless address autoconfiguration) works because routers advertise the prefix. The host hears a Router Advertisement carrying, say, 2001:db8:1::/64, appends its own interface identifier (EUI-64 from the MAC, or a random privacy value), duplicates-detects, done — address and default gateway (the RA's source) with zero server state. What SLAAC doesn't deliver historically is extra options — chiefly DNS servers (modern RAs can via RDNSS, but support varies). That's where DHCPv6 returns: stateless DHCPv6 hands out just options while SLAAC does the address (RA's O flag), and stateful DHCPv6 assigns addresses centrally when you need tracking/control (M flag), like corporate environments that must know who has what.