I got a question for you aerodynamasists (is that a word?)

When Schleicher added a bit to the wing span of the ASH-25 (25 m to 26.5m) they
required that 3 Kg of lead be mounted in the leading edge (inside the D tube)
it was to be spread down the D tube for about 10 feet and then glassed in
place.

Why? and why in the leading edge?
JJ Sinclair