The basic purpose of a normal map is to create displacement in a render.  

The depth of the displacement can be controlled using the depth of the bump map setting.

In general, if you are using a normal map for displacement, you might also want to put a specular map (if you have one) in the place where bump maps usually go. And just a very small depth setting for the combined normal and bump map. Unless you are making massively eroded rocks or something.