Why to carry spare magazines:
1. Magazines can fail.
I've read about incidents where the bottom and all the rounds fell out.
It's also possible for the follower to hang up inside the magazine.
2. It's possible to fumble and release the magazine unintentionally. (I actually did this in the presence of another member of this forum

It's much easier to reload a spare magazine that you've trained with than to retrieve one from the ground, possibly in the dark, or in mud.
3. Sometimes people discharge the entire magazine under stress. It's impossible to know whether you will do this until it happens. In that case, you need a reload.
If you want a taste of this, try the Texas Star at the Texas CHL Forum day. I was amazed how fast I emptied a magazine, to little effect the first time.
- Jim