Umm..........to compensate for bullet drop at various ranges, thereby increasing your hit probablitiy? Or are you asking why it flips completely up, so that the markings would be facing away from you if you had the weapon shouldered? That's just how it's made, probably so you can clean underneath it.
It is because the Kalashnikov series of weapons are made as inexpensively as possible and there is nothing to prevent it from flipping all the way up. It moves freely to adjust for range as Cannon Fodder stated, they just saved on the extra piece of metal needed to stop it at a certain range. Do you know how to adjust the front sight? Did yours come with the kit in the but stock?
Yes I know about adjusting for range, elevation, adjusting front sight, I know you have to flip it up to remove it, I just wondered if there was a purpose of it flipping all the way up. Thank you all for the info...I appreciate it.