Purely preference with the safety for some. Makes them feel better, I guess. With modern striker-fire pistols ---such as M&P Shield, Glock, XDs, etc.--- you already have a trigger safety and you keep your finger off the trigger until you are ready to fire, anyway. DAO semi-autos are about in the same category of a safety not being totally relevant. On a CC gun, especially, it is just another impairment to getting everything to full ready. 1911s, Hi Powers and the like, which are properly carried with a round in battery and cocked-and-locked, are another story.

Whatever you prefer to use/carry, it is best to become very familiar and comfortable with that firearm and it's safety system. The most important safety, however, is yourself.