Combat grips are designed to keep the gun from shifting in the hand from recoil in DA firing. Most target grips allow revolvers to be pushed down in the hand upon firing, combat grips eliminate, or minimize this tendency.
It depends on several different factors. "Combat grips" is a somewhat generic term that can encompass several different styles. Most, however, are designed in such a way as to allow a firm grip on the gun to minimize the movement of the grip during fast double action shooting. Likewise, many combat grips are relieved so as not to interfere with the use of speed loaders while such is often not the case with target grips. All this can sometimes, however, come at the expense of increased felt recoil and, if you have smaller hands, a more awkward reach for your thumb to the hammer.
That is not to say that "combat grips" are necessarily a bad choice for single-action shooting though. In my experience, a set of "combat grips" that fit your hands well are more comfortable for both double and single action shooting than a set of "target grips" that fit your hands poorly. Also, if you use a two-handed hold and cock the hammer with your support hand, then the reach for you thumb to the hammer is pretty much a non-issue. Finally, if you have large hands or simply don't mind having to reposition the grip in you hand for each shot, then "combat grips" would be of no hindrance for single action shooting which is primarily used for slow-fire target type shooting anyway.
I do wish when people trade or sell revolvers that they would put the target grips back on. I passed on a fine K frame this last weekend because the price was too high and it had rubber grips. But it was the missing grips that made the decision easy. Let me think, am I going into combat with a 38 masterpiece?