WMS requests let you add vector and raster data to a map. This is very useful when your main purpose is data visualization. All the rendering work is on the server side, which means that it is performed by GeoServer, and your client application just has to properly show the final output to the users.
This model fails when your users want to manage the data represented on the map, for instance, to change a style or create derivative data with a processing tool.
In this case, the original data is needed. Web Coverage Service (WCS) is the equivalent of WFS to deliver original data. It is intended to get a raster dataset, or a subset of it, in its original form, without any rendering or other processing.