The purpose is to make the link_bar more accessible after a user has read through all the comments. The user does not need to scroll to the top of the page in order to access it.
It can be tested on my goathack (links below).
Post with 0 comments: here
Post with 1 comment: here
Post with 118 comments (and resultant navcrap): here
Update: Patch available: talkread.bml.diff