It's a lot more exciting to convert a materia anticipating what materia you're going to get. Adds some much needed random intrigue to what we do, and helps seperate it from the endless grind it would be if we knew exactly what materia we were going to get.
Look at it this way, it's like opening a present without knowing what's in the box. Whilst you've opened presents before you don't know you're going to get the same thing, which only excites you and makes you want to open the present some more.
Whereas if you knew what was inside the random element and surprise would be eliminated, making opening the present a repetitive act you're not too excited about. Same deal with materia, the random element keeps it interesting.
It also stops individual items being so overpriced only Bill Gates could afford to buy them. If people found out that Dated Linen Shortgloves gave HP+70 you wouldn't find a pair cheap, and some popular materia fodder for gatherers and crafters would become unobtainable.
Possibly a poor example, since it's a common item, but you get the point I hope.
