There are probably some other uses too, but here are my best guesses. I have not really played with either spell, so I can't speak from experience. This is just what I've heard from others and/or my own speculation.
Raise land is useful for leveling out terrain (perhaps to build on), to turn water into land, and supposedly can be used in some instances to create a land bridge across areas of water.
"Turn water, beach and cliff into land; land into hill; but not land into mountain."
Lower land I assume is primarily used to either level out terrain (again most likely case would be to build on) or lower a mountain so your troops can pass over.
"Turn mountain into hill; hill into land; but not land into water."
