In C# Predicates are simply delegates that return booleans. They're useful (in my experience) when you're searching through a collection of objects and want something specific.
I've recently run into them in using 3rd party web controls (like treeviews) so when I need to find a node within a tree, I use the .Find() method and pass a predicate that will return the specific node I'm looking for. In your example, if 'a' mod 2 is 0, the delegate will return true. Granted, when I'm looking for a node in a treeview, I compare it's name, text and value properties for a match. When the delegate finds a match, it returns the specific node I was looking for.
