Why is materia capped/unusable on gear?

[Uriahnool]

Each piece of gear has a max stat value it can provide, based on its level and type. HQ gear will always be very close to the cap for all stats already on it, so adding materia of a stat already present isn't viable.

I understand this. But what I don't understand is why there is a cap, and why bother with materia if the items you get will already be capped in the areas that matter? Is materia to be considered just another way to make an item HQ then?