Yes it is a vital part of standard operations. The armrests are designed for protection in the event of an impact combined with the correct brace position and prevent flailing of limbs etc. It is perhaps more relevant in the event that there is no passenger seated beside you but at the same time some of the forces one may encounter may very well cause a passenger even fully braced to smack against another passenger causing severe hip and pelvis damage.
Now could still happen against an arm rest but at least it may protect your neighbour!