You gain the ability to more easily play cards with lots of black pips like Necropolis Regent and Vampire Nocturnus, or even Cordial Vampire and Captivating Vampire
It featured in some of the earliest decks in the game, it was a pivotal piece of the Sligh deck which essentially acted as the foundation of the mono red aggro archetype, and its presence has been significant in Modern from the first day of the format as it has featured in many of the formats best decks from Zoo to Splinter Twin to Jund
The four of them are good, and any can be the focus
A 2/5 with flying, Cyclonus, the Saboteur connives when it deals combat damage to a player, letting you draw and then discard a card, and if its a nonland card, putting a +1/+1 counter on it
If mana generated with Arena of Glory's last ability is spent on a noncreature spell that becomes a creature later in the turn, that creature won't have haste
This is great for Pauper, Magic: The Gathering's all-common format