... to fly faster. Gliders are under time pressure when it comes to competitions. They have a set course that has to be flown and documented by GPS loggers or, in the older days, by camera shots of the turning points (quite a complicated affair considering that you have to fly and do a semi chandelle and then press the camera take button).
My previous post
(upstairs) refers more to gliders, apart from the fact that I did NOT read b777pilot's question properly.
So, the glider pilot wants to fly fast around a given course (distance) in order to be home first and to be able to score the most points. You try and keep your average speed up as high as possible, depending on how strong the lift is (using a McCreedy Ring). Thus, for a GIVEN SPEED, the heavier glider will fly faster (for the same distance). The horizontal component of the lift vector is greater on a heavier glider than on a light one (speaking about two identical gliders, one with, the other without water ballast).
Then there is penetration of a strong headwind for example. Take a ping-pong ball and throw it as far as you can. Now take a stone of similar size to the ping-pong ball and throw it as far as you can. The stone will travel much further.