What is the purpose of dry aging a steak?
I go into a lot of detail regarding steakhouse quality steaks in my answer here.

Excerpt:


  Any steak you buy in the grocery store is minimally wet aged. The finest steakhouses dry age their beef. The difference? Wet aging consists of simply vacuum packing the meat (as in a whole side of cow) and refrigerating it for about a week. After that, it's cut smaller and sold to stores. Dry aging is a more complicated and expensive process. Dry aged beef is hung for at least two weeks in a refrigerator. Moisture in the meat is allowed to escape and evaporate, which concentrates the beef flavor of the beef. The beef also grows a moldy rind which is cut off and thrown away. After the aging is complete you're left with 75-80% of the meat you started with. This commands a premium price.


Please note that the dry aging happens long before you purchase the steak. The entire cow is dry aged before it is even cut. You cannot truly, safely dry-age steaks in your home. There are refrigerator aging approximations, but they are just that, approximations.