It's useful if you find that you often buy your extract per recipe. Say the recipe calls for 7lbs of pale malt LME... LME generally comes in 3.3lb cans. SO, you would need to buy 3 cans (9.9lbs) to get the 7lbs. Then you are either storing or tossing the remaining 2.9lbs. Alternatively, you could buy 2 cans, 6.6lbs, and a 1lb bag of DME and use about 5oz of the DME with the two cans to obtain the gravity of 7lbs of LME.