It's simple, really. Normal banment makes it so a user can't edit anything other than their talkpage. There's a special ban that says (on SFW) "account creation destroyed, user cannot edit own talkpage". This ban makes it impossible for the banned user to say anything on the wiki that the ban was given. Generally, letting a user edit thier own talkpage is a good way to allow them to try and protest the ban, provided they can arue thier points while still being respectfull.