Shopping can be used in many ways as game elements, something to motivate the user beyond the intrinsic learning.

A shopping environment can be a fun and familiar environment in which any activity can be placed, from learning math skills (the lower tension of the environment softens the higher tension of having to learn an unfamiliar skill) to an ironic zombie frag fest (a la George Romero's satire Dawn of the Dead.)

Earning virtual money is a great motivation in and of itself. It becomes slightly more powerful when there is a high score list, either private or public.

But shopping can also have a more direct game-play element.

Shopping can lead to buying better equipment to make subsequent levels easier, bringing simulation elements in that mirror real life and real decisions. (Buying Better Equipment in EA's Tiger Wood PGA Tour 06)

And increasingly, buying things can just be for fun and status. Ever since players could buy a giant plasma television in The Sims, the floodgates have opened. Today, walking onto a virtual multiplayer golf course with thousand dollar sunglasses sends a message to the other golfers as to your status, even if it does not help your game directly.