As I've explained previously, black soldier flies are primarily grown to provide high quality animal feed, although they can also be considered a way to get rid of food scraps.  The larvae are voracious feeders that can consume huge amounts of high nitrogen waste without any input of high carbon components.  Since you don't have to add bedding, the bins can be much smaller than a worm bin while consuming the same amount of food waste.  Even better, the larvae self-harvest, crawling right into a collection bucket so that you can feed them to your chickens, lizards, fish, or other critters. 

If you add 100 pounds of food waste to your black soldier fly bin, you should end up with 20 pounds of prepupae (large larvae, ready to change into adults.)  A nutritional analysis of dried black soldier fly prepupae consists of:

• 42.1% crude protein
• 34.8% ether extract (lipids)
• 7.0% crude fiber
• 7.9% moisture
• 1.4% nitrogen free extract (NFE)
• 14.6% ash
• 5.0% calcium
• 1.5% phosphorus

Take a look at your bag of chicken laying pellets, and you'll probably notice some similarities.  If you fed your chickens a quarter of their daily rations in the form of black soldier flies, you could take care of most of their protein and calcium needs, then round out their diet with cheap components like greens and grains.  In addition to saving money, folks growing black soldier flies for their chickens report that the feed makes their hens' eggs brilliantly orange, a sign of high nutrition.

Larvae are the primary output of the black soldier fly bin, but you do get a bit of compost --- about 5 pounds for every 100 pounds of food waste you put in the bin.  This small amount of compost can be good for city-dwellers who don't have room to use up a lot of compost anyway and who would be thrilled not to have to clean out their food composter for months at a time.  On the other hand, if you're primarily composting your food scraps to add fertility to the garden, black soldier flies probably aren't the way to go.

The other disadvantage of black soldier flies compared to worm bins or compost piles is that black soldier fly bins are relatively high maintenance.  Since you don't add high carbon bedding to absorb odors, you have to pay attention and only add as much food to your bin as your larvae will eat in a day.  However, if you're running a diversified homestead and have a bunch of chickens, that bit of time is probably worth it for the high quality feed you get in exchange.  I think that compost piles, worm bins, and black soldier fly bins will all find a niche on our farm.