budget it, do not hoard it
Each wave hands out a fixed allowance and there is no way to earn extra by playing well, so the only real decision is which sentries to put down and when. Holding back for a rainy day is usually the losing play. Anything that slips past your line goes for the crystal, and a lost crystal ends the run, so spend early and rebuild between waves with what the next wave gives you.
it evaporates when the event ends
Whatever is in your bags, on the ground or in a chest disappears the moment the last wave resolves. Stashing it in a container used to be a loophole and was closed years ago: open that chest outside the event and the stack vanishes on the spot. There is no stockpiling a head start for the next attempt.
multiplayer scales it
More players in the world means more of it drops, so a group is not splitting one person's allowance. Each defender can hold their own corner. It also has a wider pickup radius than normal loot, so you do not have to break formation to collect it.
a small oddity
Journey Mode cannot research it. Drop it into the duplication slot and the interface simply refuses, one of a very short list of items that will not take the treatment.