Why do we have bestman speeches?

I do not have a clue where the tradition of bestman speeches came from.  The Maid of Honor does not usually have to give a speech so why does the head grooms man? This is not to say that the tradition is not a good one because it is.  When the toast to the new couple is offered it can be a very warm and tender moment.  I have just always wondered what or why does it exist?  A good speech is a wonderful gift to the couple in question and a bad speech can be forgiven in time.  Sometimes a long time.