This article is part of the Maintainable MVC Series.
By using the form models as spoken about in View Model and Form Model the need for custom binding doesn't arise to often anymore.
But every now and then we have some duplicate code doing something with incoming parameters. Perhaps we can move this logic into a binder to be more DRY. Or if our form model has types that MVC can't bind automatically - like enumerations - custom binding comes into play.
MVC is extensible on the part of binding form data or get parameters to your method parameters. You can define your own binders and have them work for certain types.