I'm not sure that it can be forged. I think when Alexa first started out it may have been easy to game, but by now they are pretty sophisticated when it comes to measuring traffic.
As for use, it's just another piece of data about a domain or website to take into consideration. It might give you an indication whether a domain has type in traffic. It might be useful to see if a website's traffic stats make sense or may be forged. It can be useful to compare websites within a particular niche to see how they rank in popularity.
However, it is just one out of many pieces of data that you should use, combined with experience, research and "gut feel."