Uniform methods are an important aspect of a RESTful system, just as important as Uniform Resource Identifiers. The client of the system has to know what the methods are, so if you define your own methods you put a burden on clients to implement those methods in order to use your service. HTTP provides a set of methods that are sufficient for most RESTful services, and the methods in HTTP are widely implemented in existing libraries. Base your service on HTTP's methods and you've enabled a huge number of clients to use your service.