4How to craft it
Work Benchthe point of it
Cactus is the one pickaxe you can build with no ore and no anvil, which means it exists for the first ten minutes of a world. Walk into the desert, cut a few cactus, put down a work bench, and you have skipped the part where you need metal before you can dig properly for metal.
It opens up nothing new. The lowest metal pickaxes reach the same block tiers and swing slightly faster, so this is a bridge and not an upgrade. The moment you smelt anything at all it goes in a chest, and the jump that actually changes your day is the tier that can chew through meteorite.