Combat Celebrant may be weak in combat, but it can be exerted to gain another combat step, which is great for combo decks, hence its $13 price
Aetherdrift Aetherdrift play boosters is a tough one, because just like a race, there's only one winner and if you have 10 guilds racing like this set, nine losers
Build up a strong enough board presence, or get your opponent low enough with chip damage, and you can create a game state where your opponent can't even kill your creatures without risking death themselves
You can give creatures you dont control -2/-0 until the end of the turn, you can create a taped Powerstone token, you can make a 0/0 Construct creature that has +1/+1 for each artifact you control or you can scry 1 and then draw a card
Ugins Labyrinth does a little bit of both in Modern Horizons 3