The items, such as commands, tools, or devices, registered by a plug-in are not available when Maya is compiled. As a result, Maya has no way of determining the size of a new command object (or any other plug-in defined C++ object). This makes it impossible for Maya to allocate these objects without some help.
The creator methods on API objects provide a mechanism for Maya to allocate an object of unknown characteristics. When you register a new object, you are actually registering its creator method which Maya can then call to allocate a new instance of an object.