This barrel tightening feature is made necessary due to their takedown designs (a means must be provided so that the barrel is sufficiently tight when coupled to the receiver and yet not so tight as to make disengaging the barrel from the receiver overly difficult).
The need for a barrel tightening feature arises because of the design intended for takedown (a method must be implemented so that the barrel is securely fastened to the receiver while still allowing for easy detachment).