Not only does he shut down one opponent entirely on your turn (which is a great way of preventing a player with a hand full of control spells from scuppering your plans), but he can steal your opponents' cards right from the top of their deck
Tutoring 3-drops and putting them on the battlefield is huge, and you can get creatures that draw you cards, enchantment removal, and even planeswalkers
It doesn't affect the board, and you need discard effects to make it go brrr and fill your hand
Nissa is one of those cards that teaches a brutal lesson fast: if she resolves and you dont have hard removal, the game usually ends
Another sick way to trigger those 7+ cast effects
This price isnt due to Standard, or to casual demand, so the amount of play its seeing has kept it from being a dollar rare