One of the main uses of GeoServer is that it acts as a map server. It gets your data, applies styles and other transformations, and outputs pretty maps.

Every time a client requests a map to visualize, GeoServer has to perform a complex set of operations, load data, apply styles, render the result to a bitmap, and push it back to the client who performed the request. As your web application gains popularity, more and more concurrent requests will add and you could run out of resource to satisfy them all.

Having to build the map from scratch every time seems like nonsense, especially if your web application does not offer the user the possibility to modify styles for layers. In many cases, the styles are defined once and never, or very rarely, updated. So, your GeoServer instance will render lots of identical maps.

Indeed, when you are requesting a map to GeoServer, the chances are that the same map was produced before...