You get to draw a card each time you play a decently-sized creature, and they have trample to boot
It can let you essentially use all your lands twice, all while forcing a discard from an opponent as if the land untapping wasn't good enough
The upshot is that Expend is an aggressive-leaning mechanic that encourages players to cast their spells
Playing big, expensive enchantments with powerful effects early is a huge advantage, and being able to bring them back if our opponents destroy them is a very nice added bonus
This unlocks green and access to cool tap abilities like Kioras Follower, Ulvenwald Tracker, and Selvala, Explorer Returned
Its simple, its cost-efficient and its oh-so-effective