First of all, because the magazines are often the weakest link. Mag springs wear with use and become weak. Magazine bodies and feed lips can be bent. This is esp true if you do any hard training where mags are allowed to fall to the ground for the sake of brevity during reloads. It helps to have extra mags laying around in the event that one goes Tango Uniform.
Secondly, because in many cases, magazines can be seen as an investment. You can buy a 15 round magazine for your Glock now for under $20 in some places. If the Dems take office, you won't be able to find the same mag for twice the price within a couple years once they get another AWB in effect. Mags that you can stock up on now days for relatively cheap may be unavailable in two or three years, or very expensive to obtain. Stockpiling a few for future use, then, makes sense.
I don't have many for my handguns, or really all that many for my rifles. I have about nine or ten 30 round AK mags and about a dozen 20 round M1A mags. By the time the next Pres. takes office in Jan of 09 I plan to have a G21SF, a Para FAL, and at least fifteen standard capacity magazines for every gun in my collection. Of these, I might only keep five or six in circulation at any given time with a few set aside for the end of the world and a few others used in rotation.
It makes sense to buy mags now if for no other reason than that they will be worth their weight in gold soon. Simply as an investment, if they get grandfathered in to the next AWB, you'll be able to put your children through college with them in a matter of a few years.