Flight instruments provide IAS which is used to take into account kinetic energy for final glide calculations. A moder glider at 200km/h has at least 100 meters of kinetic energy stored. Now pull up and you get an instant plus of 100 meters on your final glide. With IAS this can be compensated and the final glide reading will stay stable during the pull up.

Another advantage is wind calculation. Best reading for the headwind component ist IAS vs ground speed, especially on ridge or wave where no circles are flown. Also usefull on long final glides with no circles.