You want it fast, you want it now
Imagine if, for every meal you make, you have to go to the grocery store and purchase only the food you need for one meal. If you forget a key ingredient, you have to return to the store. You also have to throw away any leftovers. You don't have a refrigerator or cabinets to store any items. When lunch rolls around, so does another trip to the store. The same holds true for dinner. And because everybody must make a trip to the store for every meal, the store can become rather congested.
Oh yeah, and in your kitchen, all you have is an open flame and a single spoon with which to cook. Not very efficient, is it? However, this is basically how today's thin client applications work. Nearly all resources exist at the server (the store), while the user must make several time-consuming attempts to access those resources, only to be used once in a tightly controlled, limited manner (basic form entry/submission).
But what if Web applications worked in the way you actually operated in the kitchen? You go to the grocery store once a week, purchase a wide selection of items, and then bring those items home for your immediate use throughout the week. You have a wide assortment of tools that make cooking not only easier but also more enjoyable.
You can implement a Web application based on such logic through a rich client, where the application is optimally dispersed across multiple tiers. Sets of data, business logic, and highly interactive UI elements can be downloaded to the client for instant user interaction. The result: the best of both worlds. You have the deployment ease of a networked application and the efficient functionality of instantaneous response.