While the first inclination might be to use a base texture as its own detail map (greyscaled to enhance what is already there), the use of another completely different texture can be made into a detail map to expand upon the diverity of the base texture with out having to alter said base.
For example, I have been making a desert world for ELSEWORLDS!.
I start out (just for the examples sake) with a http://i57.photobucket.com/albums/g235/Wesadam/Texture%20Tips%20and%20Tricks/DMAP_1.jpg (which I use for floors and walls where needed).
Then, I apply my very simple http://i57.photobucket.com/albums/g235/Wesadam/Texture%20Tips%20and%20Tricks/DMAP_2.jpg to the plane.
Now while this is good for say, floors, I want to expand on the texture and make that match, but not exactly (so as not to
give the player vertigo while travelling trhough a sandy tunnel.
So, I decide that I want to make the sand appear as if it were carved like stone. I then find a good stone texture that
will alter the texture to my satisfaction. http://i57.photobucket.com/albums/g235/Wesadam/Texture%20Tips%20and%20Tricks/DMAP_3.jpg.
I take the stone texsture, convert it to gray scale/B&W, and then dupliate it in a new layer. I make the top layer a negative and make its opacity around 46-47% (doing this will give a nice even grey tone that is preferable at first). Then, I add a small emboss filter to the layer beneath and blur it a bit. I then focus/sharpen the top layer in the opposite percentage that I blurred te embossed bottom layer (this is just my method, so insert your own method here
). Once all of this is done, the new detail map texture should look http://i57.photobucket.com/albums/g235/Wesadam/Texture%20Tips%20and%20Tricks/DMAP_4.jpg.
Then, I apply both the base texture and the detail map to obtain http://i57.photobucket.com/albums/g235/Wesadam/Texture%20Tips%20and%20Tricks/DMAP_5.jpg; a very nice sandstone to compliment the sandy floor.
For added effect, one can adjust the contrast and brightness of the detail map for even more depth (as the subtle grey is gives an even lit tone, but in a higher contrast version, what is close to white acts very much like a glow map and what is close to black acts like dark map, resulting in a more natural reaction/imposition of light on a mesh).
Now while this might seem like and extra texture (regardless I would have had to make another texture for the walls in some fashion), the detail map can be applied to other base textures for even more variety with (actually) less textures
(which is always a good thing for smaller downloads).
Now mind you, this is just a quick example as to what can be done with detail maps, if one is creative enough, then they are a very handy indeed
.