When sailing in wind that is so light that it can''t keep your sails full, inducing the boat to heel by moving crew weight onto the lee rail helps keep the boat moving. Gravity causes your sails to hang in the curved shape that drives the boat, instead of hanging limp, like a sheet on a line. Also, moving crew weight forward and to leeward usually reduces the amount of wetted surface. By heeling the boat, the outboard end of the boom is less likely to swing in light air.
It takes a lot of crew weight to heel a big, ballasted boat, but the benefits of heeling more than offset the added weight. If you give the sails a full shape and bear off the wind a bit, you can keep moving when others are losing steerageway.