The Monster Juicer is useful when experimenting different attack strategies. Build it right after unlocked. Do not sell it or you will lose the ability to remove monsters from Housings.
It still costs 1M Bones, Coal and Sulfur in the Inferno so hatch monsters carefully while you still haven't built it. You can transfer it to your Overworld Yard if you have enough space and then juice it there.
Upgrading the Juicer will increase its refund %. Upgrading it to level 3 will give it 100% refund. You can use this to get free resources by juicing monsters and collecting the quest rewards. Pokeys are recommended when completing these quests since they only take a few seconds to hatch.
The Monster Juicer can juice champions. This is not useful anymore since they don't give any goo and you can just keep them in the Champion Freezer. This function can be used to cheat using a level 1 champion.