The idea behind this is to create infinite tokens that can be sacrificed to inflict damage
Adventure cards are two spells attached to a single card where the adventure spell may be cast from the hand and then the card is placed in exile
Its true that those zombies enter the battlefield tapped (its safer to cast it when you have enough blockers to protect you), but using one card to get from 13 to 26 2/2 black zombie creature tokens is terrifying
The copies are created on the stack, so it's not "cast." Creating the copies won't cause abilities that trigger when a player casts a spell to trigger
It isnt much to go on, but if you see someone with, say, a large amount of Unglued Plains, Mountains, Swamps, and Islands for trade but no Forests, alarm bells should ring in your brain