I was trained to speed up for the approach in gliders because, if you suffered a wind gradient at low altitude
pulse1,
The primary reason for flying faster on approach is because gliders, particularly trainers, are often flown slower than a safe approach speed (1.3 Vso). Then on top of that, an additional increase in speed can be required, depending on the wind velocity, to allow for wind gradient effects, just like in a powered aeroplane. For example, the SAC Instructor's Manual recommends an approach speed of 1.3 Vso plus the full wind speed.
http://www.sac.ca/website/index.php/...rts-a-a-b/file