Because if your account gets hacked and you lose your password you have to prove to theymos you own your account.

You can only do that by signing a message from a bitcoin address that you have previously either posted here or sent to someone in a pm.

I think it's now against the rules to post an address, so you need to pm it to someone.

You only pm theymos after your account is hacked, and prove to him you own that bitcoin address so he can return control of your account to you.