[May 26, 11:11:15 PM] ADirtySock says:
They’re used only for infusions, which you can do by swiping left on any gear piece if you’re on mobile, infusing upgrades any stat of your choosing by one and the cost increases every time you do it.
You should save all your materials until you get a good set because it can get quite expensive but most of the end game gear you should be using will already be infused by the community so do whatever you want.
Also, when you look at a gear piece, the little numbers in a white box by each stat is how many infusions are on that stat.