Why do so many dessert/biscuit recipes have salt in them?
Salt has unique properties in how it interacts with the taste buds. While it has its own "flavor" it also has the ability to enhance some flavors while blocking your ability to experience others.

While I could go on, all I would be doing is repeating much of what I learned watching 
The Food Network's Alton Brown. He goes in depth for the episode "The Ballad of Salty and Sweet" which is available on from the Food Network.

There are (of course) other resources, but AB does a really great job of teaching in this episode.