yea they come in really handy for breaking up large tiled sections and can help you get the most out of a smaller set of tilable textures.
usually they are just black and white, so for example in unreal you can have a texture that has different grunge maps in each of the r g and b channels so its 3 for the price of one.
That's what we usually do for 3d engines: artists make tileable/wrappable/mirrorable texture ( just using Photoshop's offset filter + stamp tool ). The game level designers place the textures on walls tiling them several times(5, 6, 7 times). Then, some decals are placed ( graffities, dust, marks, etc ).
At Mythic, we usually have a dark map texture that is mapped to a different UV channel then the diffuse that works like a mulitply layer in photoshop we also have detail maps that act like overlays.
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usually they are just black and white, so for example in unreal you can have a texture that has different grunge maps in each of the r g and b channels so its 3 for the price of one.
I hope it helps.