The Skull Ring is a good card to use if you have a specific strategy in mind and want to condition the cards you get. At 2 energy (1 black and 1 normal), it is both useful and easy to play. For free, once every Main phase, you can see the next card coming up in your deck and choose to send it to the graveyard or keep it where it is. You can use this card to get allies in your graveyard in order to play something that requires them, like Horrid Skeleton.
Skull Ring can also give you allies for the Bonemancer and Monster Mage, or pick off cards you don't need. If you need a specific card for a strategy, discarding one card from the top of your deck gives you a better chance of picking the card you want.
Strong against: Nothing in particular.
Weak against: Object removal, milling (you'd end up helping the enemy empty your deck by getting rid of extra cards yourself).