There is nothing wrong with a hammer, nail and nail set. But it really doesn't compare. It isn't just the time putting the nails in. It's the time putting them in accurately, sometimes pre-drilling, filling the holes with filler, tossing out a few bits of trim because you put a few awful donkey kicks into them, etc.
If you're putting on one piece of trim: hammer, nails, nail set.
If you're putting on a whole house of trim, or fighting gravity putting large bits of trim above your head, or doing jobs that need 3 hands to hold things in place while you nail: there just isn't a substitute for a nail gun.