Adding weight to flies can be tricky. A lot of tiers prefer to simply add a bead of varying weight to the hook before tying, which not only gives the fly some weight, but also some flash and character.
But sometimes, as Tim Flagler notes in the video above, adding lead-free wire to the flies not only helps with weight, but it helps build up a body for a heavy nymph pattern.
As a note: Tim only ties with lead-free wire. Lead is toxic to both fish and waterfowl, so this is just simple environmental responsibility.
— Chris Hunt