Arcbound Ravager is simply supremely effcient when surrounded by artifacts and artifact lands. He does quite a few things, and all of them for zero mana. After playing Magic for a time, you should realize how powerful zero mana abilities are. Examples of zero mana abilities that make for powerful cards are Psychatog, Wild Mongrel, Ravenous Baloth, Mogg Fanatic, and more (Quirion Ranger actually can generate mana with her ability).
Ravager turns any artifact that would die for any reason into a +1/+1 counter. Ravager at any time can turn all your artifacts into +1/+1 counters. Ravager at any time can move all its counters onto another artifact creature.
I've played against the Ravager Affinity decks. They kill you turn 4 every time if a Ravager hits. You block the Ravager, then he sacs his entire table, you lose 7 life from Disciple of the Vault, and the Ornithopter grows huge and deals the final 7 to you.
If it cost 1 mana per activation, it would be several turns slower. Ravager more or less works like Psychatog does, but Ravager needs lots of sac fodder where as Psychatog needed you to draw cards and play spells during the game.