FreeNAS is what experienced/wise long-time unix admins will setup at home for their giant anime SAN/NAS because:

- they've been around the block long enough to know that FreeNAS can do 95% of what they need better than they can do it, with far less chance of errors introduced, but with 95% less of their valuable time needed to do it

it's that simple honestly.

if you already know how to do setup/configure/tune/tweak all of the components inside FreeNAS specific to your environment, congrats, you're already a great unix admin. you've been in your career a while, and you're probably pretty well paid.

but why would you spend your valuable per$onal time to faff about setting up/configuring/tuning/tweaking what FreeNAS can do out of the box, unless you have an edge case scenario that FreeNAS doesn't cover or can't do well?

for the same reasons, again i must specify -- unless you have very specific edge case scenario(s)/situation(s) or wanted to use this as a learning exercise -- why would you:

1) hand-craft your own *BSD or Linux-based firewall/VPN/router, instead of using pfsense/iptraf/etc

2) hand-craft, build and create your own Wifi router OS image, instead of using OpenWRT

3) roll your own port monitoring and OS proc monitoring notification system, instead of using Nagios/BigBrother/etc

so to get back to your OG question of

> If I already know my way around FreeBSD and around a command line, why would I want to use FreeNAS instead of just sticking to vanilla FreeBSD?

because your time is valuable, and because FreeNAS can do 95% of what you need better than you can do it.

(edited for formatting/clarity)