What is the use case /scenario where should I create an EndPoint in google app engine project?
Endpoints should be created on a class basis. So you use the same endpoint class for sending and receiving foo objects (but obviously you use different methods). It's just a REST api, so you'll want to create your basic CRUD methods. Generate an endpoint in Eclipse and you'll see what the intended/generic use cases are.
Is it advisable to have same EndPoint to be used for mobile client and web client?
Yes, that's the point of a REST api; it doesn't matter what the client is. From the overview page: