I usually only lock mine for if I'm redecorating, or if I go through periods of time where I log in and there are random people in my house just like hanging out or...doing other things. I actually walked in once on two people ERPing in /say and I was like "uhm...." and locked the house for a while after that lol.

But I also like privacy, so that's another reason. When I was in my old FC, I always kept my FC room locked if I ever wanted somewhere to craft or just idle in peace without a bunch of bustling or whatnot around me.