The diopter adjustment (a dial or, sometimes, a lever) adjusts the viewfinder for less than perfect eyesight so that one may use the viewfinder without wearing glasses. I do not believe that it works in all cases, say astigmatism or extreme short range vision problems.
It may be found with both optical and electronic viewfinders - I have one of each. If you do not need glasses, then read no further.
If you do wear glasses go to any large camera store and ask to look at cameras with that adjustment - the Kodak DX7440 is an example of one with an optical optical viewfinder, the Canon Optura 100 is a mini DVcam is an example with the EVF.
The diopter adjustment is very useful to me but I think that's a personal thing and am not sure that I would let it over ride other wanted features.