The heavy eyes, made of brass or tungsten, lend themselves very well to tie streamers. Tying them to the hook is very easy, you just need to fix them with the Tying thread through simple crossed windings.
With these ballasts you can practice a proper fishing action in all those situations in which you need to sink your artificial flies very quickly, to reach the bottom of deep holes or to fish in strong currents, also weighing down your streamers will make it much easier in the long cast.

Some of the heavy eyes you'll find in this category have a groove in which you can place adhesive eyes to make your imitation even more truthful and attractive.