They are used as a cheap manure for the land

Edward Donovan, The Natural History of British Insects, Londres, imprimé pour l’auteur et pour F. et C. Rivington, 1792-1813.

The phryganea undergo their transformation in the water: in the larva state they are taken by the fishermen for bait and in some parts of holland are found so abundant, that they are used as a cheap manure for the land.
In the larva state, they generally form a sort of covering or tube, for their defenceless bodies it is open only at one end, at which its head and fore legs are protuded, to take its prey. Some species form these coverings of weeds and small shells, gravels, sand, etc.
That of our present species is composed of little pieces of the stalks of grass, cut into an even form, and laid transversely on each other. It attaches this tube to the roots of some aquatic plants, and undergoes its transformation in it. In the annexed plate, we have represented the larva taken from the tube and the pupa having the tube opened to exhibit its situation therein.