As far as I know the purpose of the stripper is to strip the turbulent air from the back of the pellet thereby increasing it's stability.
When a pellet is launched down the barrel obviously the air pressure behind the pellet is much greater than the pressure in front of the pellet. At the point of the pellet leaving the barrel the high pressure behind it is released into the open air (hence crack from the muzzle). The high pressure air expanding in all directions just behind the pellet can destabilise it. The theory is that the cone in the stripper strips the turbulent air away from the back of the pellet before it can destabilise it.
In reality I have found no appreciable difference in accuracy between and open barrel and air stripper and a moderator but the 'pro-stripper' brigade tell me its because I have never set up my stripper properly.
So many guns, so little time