Like many utility tap lands, Mortuary Mire comes into play tapped. But it has an enter the battlefield ability that interests a lot of people. When you put it in play, you may take a creature card from your graveyard and put it on top of your deck. Imagine you could draw into a creature that you could really use from your graveyard. This is exactly what Mortuary Mire allows you to do.
Now, in Standard, letting your opponent know what your next draw will be isn’t always optimal. However, in Commander, a format in which you’ll be likely very able to draw that creature before your next turn, a card like Mortuary Mire is invaluable. In Commander, you only have one of each creature, so being able to get back a key creature and put it right back into your deck is a big deal.
Mortuary Mire sees a ton of play in Commander according to EDHREC. The top Commanders that use this utility land include Sidisi, Brood Tyrant, Ayli, Eternal Pilgrim, Meren of Clan Nel Toth, Phenax, God of Deception, and Gisa and Geralf. None of these Commanders are particularly surprising to see, as Mortuary Mire helps get back key creatures in these decks. Plus, getting back a creature on a land that gives you colored mana means that you can get a lot of value out of this one card.
Always look for utility lands with decent effects like this that provide one or more colors of mana. Many utility lands only provide colorless mana. While it would seem to limit their usefulness, utility lands that provide colored mana are usually pretty good, especially in Commander. Whenever you see a copy of Mortuary Mire lying around, make sure you keep it within reach. You never know when someone might need a copy.