It depends on the kind of silk
If your silk is larger, it will have more industrious uses. Especially if it is large. Things like rope, tents, and other situations in which animal hides or leather would be used. Maybe they'd make saddles or boats out of it.
If it is smaller, and smoother, it might have uses such as clothing, or bedding. It would be like thread, although probably thick thread.
The eggs could be eaten, and that would work as well. These creatures might also be eaten, or something similar. They could be grown for their hard hides, which ae like stone and can be used to make tools. Don't be afraid to implant other uses in your mites, to make them more beneficial.