I don't know what Glock model you are referring to exactly, but some models have grips that are too short to fit an average-sized hand, so the pinky finger just dangles, which bothers some people. So people use longer-than-standard magazines (which generally hold one or two more rounds) to allow a full grip.
Second, many people have a specific shooting routine at the range. For example, they shoot a group of 20 shots in four 5-shot strings. If a handgun comes standard with a 6 to 8-round magazine, which is common, then you need to have at least 4 magazines and you need to reload during the 20-shot string. Again, sometimes this bothers people, who would prefer to use two 10-round magazines or one 20-round magazine. This may seem trivial, but if you shoot regularly (say every week), you would probably consider using larger-capacity magazines for convenience
Last, magazines are designed to hold a certain number of rounds, which they typically do. However, in some states it is a crime to sell a "large capacity" magazine to a non-government employee. Therefore, when the law has a 10-round limit, and the magazine is supposed to hold 10 rounds, you often find that not only will it not hold 11 rounds, but that the 10th round is almost impossible to squeeze in, which is a pain. So again, people who want to shoot 10-round strings (see paragraph above) will sometimes get +1 or +2 extensions just to allow the 10th round to fit.
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