In many cases, it's due to the availability LME is cheaper, but sometimes it comes only in 3.3 pound cans. If you need, say 4.5 pounds, for a recipe, it's hard to use 1.45 of a second can. So, you can use DME by the pound to "make up" the difference.
Also, sometimes you need a specialty grain that only comes in LME. For example, your recipe might use Munich malt as 1/2 the fermentables. Since it only comes in LME, you might use 3.3 pounds of Munich LME and 3 pounds of DME for the recipe. DME comes in "extra light" so if you want a lighter beer, you'll use DME for some of the fermentables.