The advantages to a belt mounted (especially IWB) mag carrier:
1. Same position no matter what pants. Therefore quicker/easier to access. Many pants don't have "watch pockets".
2. Same height. Easier to access. Watch pockets vary in size and depth. The mag is not always in the same position.
3. Secure, even in a fight when rolling on the ground. Pocket friction is not the same as Kydex friction.
4. Can be positioned on side opposite gun hand. Watch pockets are only @ 2 o'clock.
5. More concealed. Exposed part of mag is above the belt rather than below.
6. Cartridges are retained securely, protected in the mag carrier. Cartridges sometimes eject from a mag when jostled in a pocket.
7. Less wear and tear on the pants. Pocket carry allows more dirt to get in a mag, and the mag wears out the cloth of the pocket.
So, watch pocket mag carry may seem like a cheap, easy, simple way to carry an extra mag. It is, just like a floppy generic nylon holster retains a gun. There are better, more serious ways to prepare for a life threatening event. They cost more, but they are worth it.