Why you should use NoScript
Sounds simple, right? Well, NoScript has a lot more features and options, feel free to install it and take a look at its documentation and features, it is really worth it.
Well, the answer is for security reasons. Exactly as when I do a network firewall setup, I prefer to go with a block by default, then allow what is needed policy in my browser. I think this is a very good idea, specially if you take in account things like this bugs here: