Dredge is such a cool deck because it defies all the regular rules of magic. You get free stuff, creatures enter the battlefield from places that aren't your hand and, when you have a really good hand, the deck produces sooooo many creatures so fast that your whole playgroup will be marveling.
Let's take a quick look at the non-dredge creatures we're using in the deck and how they function in this strategy.
Starting off simple, let's look at Bloodghast. Keeping in mind that this deck revolves around filling the grave just consider that all it takes to get this creature into play is a simple land drop. With 2-3 Bloodghasts in the grave this becomes quite potent.
Now, let's look at something a little more complex: Ichorid. This creature is meant specifically to trigger with another card in the deck, Bridge from Below, which I'll talk about in further depth in a moment. Ichorid requires exiling another black creature at upkeep to receive a 3/1 haste that sacrifices at the beginning of the end step. He's powerful, but he doesn't stay around long.
Now consider Bridge from Below - the card that was mentioned to interact specifically with Ichorid. Basically Ichorid will enter the battlefield and attack, thereafter either dying or sacrificing. What this does is when he (or any other creature for that matter) enters your graveyard you get a 2/2 zombie for each Bridge from Below in your grave.... Now, consider that on turn 3-4, which is quite late in the game for a deck of this speed, you have three Ichorid and three Bridge from Below in your grave, the end result is nine 2/2's... It's a big problem for the opponent and also one of the things that makes this deck such a riot.
Now consider Prized Amalgam. This creature is a freebie if you've had another creature enter the battlefield from your grave this turn. Well, the first two creatures we've talked about, Bloodghast and Ichorid, both trigger this ability. Basically, as you can see, this is now a group effort and we more or less have creatures pouring out of the grave and onto the battlefield waiting to create chaos.
Last, but certainly not least, we have our little jellyfish, Narcomoeba. This card is weird as hell but is yet another freebie for the deck so it's an auto-include. Basically, as you're dredging apart your library these guys will just fly off the top of the deck and onto the battlefield for free. They're nifty, but once again weird as hell. I love them.