This means it's not possible to cheat creatures into play out of nowhere (which madness can do), but it also means you're granted more flexibility, because you can cast a spell for its mayhem cost at any point during the turn in which it was discarded (rather than having to choose immediately as per madness rules)
If youve completed a dungeon, that creature also gets +5/+0 and gains first strike and menace until end of turn
While it might seem enticing to simply cheat in big, huge power and toughness creatures, Reanimator decks dont win by just turning cards sideways (usually): they win by bringing in back-breaking cards that do many things upon their Enter the Battlefield effects
The theme decks are also where you can find some error cards, No Damage Ninetales, Ghost/Phantom Pikachu, and Sideways Fighting Symbol Diglett
You say sorry a lot to counter spells and stuff