Dwarven Armory is one way to turn excess land at the end of the game into a playing advantage. Usually you'll have too much land at the end of the game anyway, and with cards like Thawing Glaciers, you can always get more if you start running low. By sacrificing just one land, you can place a +2/+2 counter on any creature. What's so great about that? You can instantly make any creature Timmy-proof, Reveka-proof, Skull Catapult-proof, and if it's got a toughness of at least 2, Bolt-proof! It's really fun to put the counter on a first striker...a first striking 3/3 can take out a Juggernaut, and a first striking 4/4 can take out a Craw Giant! Or, you can always put a counter on a Dragon Whelp and make him big enough to kill a Serra or a Vampire and still survive. And just imagine the fun of a 6/6 Sengir!

There are some less obvious combo's with Dwarven Armory, too. For example, you could prepare some of your key 1/1 creatures for the Pyroclasm you are about to cast. Or you could even pump up an opponent's creature, then cast Reprisal or Retribution of the Meek. Or use it to boost walkers. Helva loves to use it on Veldrane of Sengir when I'm playing forests. It has millions of household uses!
—Ulrich