Risk management.
Quite apart from gpn01's valid point about which is the most appropriate ATC unit (and in this case it's information - they have no controlling responsibility) who is most likely to give a glider pilot useful information - a wide area Air Traffic service or another glider pilot in the same cloud with a personal interest in avoiding a collision?
And as for knowing where we are, when racing in competitions control at a waypoint is recorded by GPS position within 500metres of a point. Looking at the positions competent pilots fly just into the circle (typically 450meters from the point), tight turn and fly on. I know my position to within a wingspan (actually better - my normal span is 26.5 metres).