Whats the benefit of z-stack

Hey all, I'm new to the dbs card game and I'm wondering what the point of z-stack is. I have Mai, Together With Trunks and I don't get the purpose of using its z-stack effect. My understanding with z-stack is that you're allowed to place up to X minions under the z-card with z-stack and if the topmost z-stack card leaves the field, all the other cards in the z-stack goes into the drop zone.

thank you