There is more to it than just "more amo". This is WHY people should seek out skilled instructors.
For one, with most semi automatics a quick way to recover from a FTF is to drop the mag and insert another full one.
For another in the event you HAVE to use your fire arm, how many did you fire? How many do you have left? Are there any MORE threats? Do I have enough left to make my self aware of any thing else?
A fast, easy, dead positive sure way of answering all those questions is to drop the mag and insert a fresh one before clearing all threats at the scene. When you look around and it looks clear, THEN pick up what is left of what you dropped and put it in your mag carrier. Holster your weapon, and call the police.
There are MORE advantages available if you have an extra mag, but just those should be enough to make a guy think hard about it. It is for me.
One really handy thing to know, practice, be aware of with many semi-autos. IF YOU WERE EVER to find your self fighting an attacker for control of your weapon and fearing you are losing. You could possibly drop the mag out by hitting the button rendering it in operable should your opponent gain control. Yet if YOU regain control you can immediately bring it back in to the fight by inserting your SPARE mag. IF the mag drops out on the floor, many criminals may even STOP fighting for something they consider rendered useless.