glMultiTexCoord4f

Whenever you want to set the s, t, r, and q texture coordinates while using multi-texturing.

and a more modern use, if you're using a fragment program and you are running low on space to pass things across then you can pack two texture coords into one and decompress them in the fragment shader.

its also often used to supply vectors to gl, eg i use
glMultiTexCoord3f to supply the tangents/bitangents/normals to gl.
ie it can be used for anything where u need numbers, they dont have to be just texture coordinates