I use theri to gather wood and fiber, then craft primitive bows at 10xp eaxh and feed them to my gachas. 1-3 bows=crystal plus helps level your character fast below lvl 70. Will poop out rare items still too.
Gachas are essentially living lootboxes and resource farms. They will eat anything (including stone, wood, or fiber) and turb it into crystals which can drop a variety of resources including black Pearl's, organic polymer, or obsidian, among other things. They also have a rare chance to drop higher-tier crystals which turn into upgraded armor and tools