Back in the old days, when we had to use smoke signals) after walking to our computers in the snow uphill both ways...well you get the picture....we didn't have a side bar like we do now.
We could tell that something was hot because when you scrolled down the "messages" thread it would have lots of responses. So each time a thread is responded to, it goes to the top so that others just logging in can see it.
So, if you had not relevant comment but wanted to alert other freepers to the news you would "bump" the thread or shorthand bttt form back to the top.
For nonbreaking or old threads it can still be used to call attention to a thread.
I rearely use the sidebar. The latest message queue is how I keep up, so the bumping of threads is important to me.