Some are worded similarly to enters the battlefield triggered abilities, but say a permanent enters with or enters as
Its controller may search their library for a basic land card, put it onto the battlefield, then shuffle
Even when we aren't comboing off, Mirrorpool gives us some odd value by copying Manic Scribe (especially in the late game when we have delirium) for additional milling, or sometimes just copying a Compelling Argument is enough to get our opponent out of cards
That will fit a lot of decks, but what happens in the next chapter is that you get two rounds of a pseudo Council's Judgment, and that's a pretty strong effect
If you exile a land card, you can play it only during your main phase and only if you have an available land play remaining
This is a lot to pay for a built in shuffle but due to the power of putting cards in hand back in the library the Legacy sees a vast amount of play and remains one of the best card quality cube cards