Well, you put things in it that you want to cook in boiling water, and then remove them all at once without fishing around or draining.
So, you could have a large pot of boiling water and have different vegetables in different bags. Cook them all in the same water, but remove each separately as they're done.
Who cooks like that? I really don't know. But you could.
You could also put a chicken carcass, aromatics, and vegetables into the bag and it would all hold together when you make stock. Then, just remove the bag. The stock would still need to be strained to get out the odd small bits, but the majority of the mess would remain in the bag.
It can also be used for foods that you want to remove from boiling water and then shock in cold water.
I've tested it with tomatoes and eggs and a few other things, and it works like it's supposed to, stretches to hold more than it looks like it does, and cleans up easily. It was slightly easier remove cooked eggs in one fell swoop and plunge them into cold water, but it wasn't a life-changing difference.
I'm guessing there are other people who would find this indispensable. Or perhaps there's some genius use I haven't discovered yet. I'm certainly open to suggestions.
After I posted this on Serious Eats, I got an email from a visually-impaired reader who said that it could be very useful for cooks like her. Now, that makes a lot of sense. So, you see, just about every gadget is going to be very useful for some folks.
The product was supplied for the purpose of a review on Serious Eats; this was previously published on Serious Eats.