If you cast a Bolas, you can turn it into removal
The cheaper and better ones like this do so the most and it makes the biggest difference to singleton formats where redundancy is such a tough thing to attain
Conspiracy allows you to choose a type for all of your creatures, while Maskwood Nexus turns all of your creatures into every creature type
This is something all players want, the sure and certain knowledge that no one is going to mess with their creatures, spells, or plans
If you let them use Aether Vial to generate mana, they can start producing things that get in the way faster than you can answer them
Bane of Progress is particularly good with Ygra, as the former will get a +1/+1 counter for each artifact and enchantment destroyed and the latter will get two +1/+1 counters for each artifact that hits the graveyard