Cards can be changed if theyre too powerful (either by having a higher mana cost or by having abilities removed) or if theyre not powerful enough (either by having a lower mana cost or by having abilities added)
Many let you draw cards after certain acts or just by playing, which means you can get through the deck fairly quickly
Players usually can only play one land card per turn, with most land providing a specific color of mana when they are "tapped", usually by rotating the card 90 degrees to show it has been used that turn
Between milling yourself, discarding a card, and sacrificing the saga, it pushes tons of cards into the graveyard to fuel your graveyard strategy, while the removal chapter gives you time to exploit those resources
Its possible to get fuel for your hand and for your graveyard, and its not a bad deal even if you pay 4 mana