Glass beads are very good if you want to fish light. With these beads you can make your nymphs work just below the surface of the water. Surely in most cases you will have more success fishing in close contact with the bottom, but if you can read the situation correctly and understand what is the exact moment when the trout are feeding in midwater, you will have excellent results using the glass beads.
At certain times, in fact, the trout are concentrated on preying on the adult insect in the ascending phase, which comes off the bottom to reach the surface of the water. In those cases the fish will tend to be disinterested in anything else that does not enter its field of vision, making both bottom fishing and dry fly fishing in vain.