Well start with reasons we currently ban cardsbut Ill remind you that the reasons arent hard and fast objective measures
Pick up a playset of four and keep them
Sliver decks are based around each card granting incremental bonuses or individual keywords until you have an army of creatures with haste, +1/+1, vigilance, poisonous, trample, reach, and more
If you discard a mayhem or madness card, you can cast it for its mayhem or madness cost
You can check out my cube over on CubeTutor here:
If we cast and +2 a Nahiri, the Harbinger, on the next turn cycle, we can +2 again and use Glorious End to make our opponent skip their turn