In MTG, a card or ability that functions at instant speed can be played at almost any time, including during an opponents turn or in response to another spell or ability
Exile all lands you control of that type
By allowing it to target your own spells, you can suddenly build around the random element of this card and fill your deck to the brim with obnoxious expensive cards
I hope you enjoyed a little peek into my "hand" of cards
Fertilid, Wood Elves, and Springbloom Druid all fetch lands out of your deck, just like Rampant Growth, but you can flicker or reset them for additional value
Put any number of those permanent cards onto the battlefield, then put the rest of the revealed cards on the bottom of your library in a random order