since we cannot see whether it is merely further in and out or higher and lower, i will assume it is higher and lower - as in most of these that i have seen
i thought as Remi that is was for wind strengths and leech help
however, i have seen the pros discuss it and have seen experiments first hand where
the lower clew is better for "shorter" riders and the upper one for "taller" riders
AND you can even rig in-between
thus you actually have THREE(3) settings and NOT just two(2) !!!
try them and see which suits you best and perhaps even under which conditions