Take an image and try reducing exposure and compare the effect to bumping up the gamma. Lowering the exposure seems to make it look more dull, while bumping the gamma seems to make a crisper darkening. I suppose the technical reason is something about contrast change difference between the too methods with gamma raising and lowering contrast as well as darkening and lightening. When I want something brighter, I usually like the effect of raising the exposure. When I want something darker with increased contrast, I prefer to bump the gamma. Bottom line is I don't feel the need to intellectually understand how gamma works. I just try different ways until I develop an intuition about what the controls visually do and work from there. Occasionally I experiment when I'm unsure. It's quick and easy to undo if I don't like the effect. More play time!
