but a message saying "subscribed" is a +1 to post count, haha, why else do you think they do it?

 

also it's a friendly bump to the top.

 

Is there a "subscribe" link that is available without clicking to another page first?

 

It is done to quickly subscribe to a thread of interest, bump it to the top, and put a little blue mark beside it whenever it is viewed on the front page again.

It also lets the OP know that, while there may not be many replies (yet), that many people are interested in the thread, even though they may not have anything to add to the discussion.