Hello TommyT,
The leg angle also changes the basic elevation of the muzzle. Depending on your shooting position and rear bag height (if used), sometimes you want it higher or lower. If you're shooting at a target below you in elevation, you want less height from your bipod and vice-versa.
When I'm filling my PCP and want it to stand by itself, I need to point the legs straight down, for instance, to keep the pistol grip from hitting the ground.