Bumpmap = Greyscale(Texture) ???

while searching the web, I’ve seen lots of times some sites
trying to sell greyscale versions of textures as corresponding
As far as I understand it, a bumpmap is a completely different
information, otherwise it wouldn’t be neccessary to set up
a separate image file. (And a dark spot in the texture does
not neccessarily correspond to a dent).

Nevertheless I tried those “bumpmaps” and found that the
resulting renders look much better than when not using those

Now my question: Is this common practice to use such
“bumpmaps”? After all, maybe I’m wrong and either
the supplied images are not just the greyscale ones,
or there is some explanation why it looks better?

Best regards,

Bump maps are indeed greyscal eimages. The depth/height of the bump is determined by the resultant amount of white or black in the image. White being the highest and black being the lowest. Thus, grey areas and varying degrees of light/dark are interpreted as being less high/less deep.

So indeed, using greyscale images will result in much better results with bump maps.