In addition to all of these answers: end game gear doesn't always make high level materia. It's good to have crap materia and socket it to good gear, because it's better than nothing. As you attain better materia, you can have the crap materia removed by NPC and attach the better materia. It's a good deal, because crap materia won't fetch you a huge price (but it will get you something, as others are suffering from "no good materia syndrome" as well, and will want to put something on).
Given the number of short answers in this thread: