It's a DOS emulator for modern systems. When I play MM3 (or Master of Orion, or Master of Magic ...), I can do that comfortably on my main box just because of that.
Moreover, I believe there are various advantages of using DOSBox over native DOS. It can, for instance, emulate inserting floppies. And it handles sound correctly (I don't really remember DOS, but I believe lots of these "goodies" were pretty random --- the sound either went OK (if you were really lucky), or it made unbearable noises instead of sound (if you weren't), or it didn't make any sound at all (if you weren't).) Etc.