Confused about membership provider and what else I need to use..

I am creating some inhouse web applications(MVC2) for our employees to use..
We currently have a separate backend system that stores login information, session ids(which get stored in browser cookies), ect..
I am trying to create a custom membership provider to work with all of this..
I have seen some code samples where people just set a reference to and create an instance of their custom membership provider and then just executing the functions wherever needed (Eg. ValidateUser).. If thats correct then whats the point of inheriting the membership provider class, why not just create your own class to do what you need with your own method names.. Also why does the membership provider need to be referenced in the web.config file if its just a class that is used from the controller????
Is there something im missing here or maybe more that I need to do??
Also is it possible to make my custom membership provider global so that any we ...