I see this a lot in olympic weightlifting. You'll have some full diameter plates, then the collar, and then some smaller, for example, unit 1kg plates on the outside. Is there a technical reason why this is done? That is, why not tighten it in with the collar?
From personal experience from a couple of gyms (and admittedly this may not always be the case), if the plates are smaller rubber ones, then they actually fit very tightly onto the bar so despite the dynamic nature of the Olympic lifts, there's virtually zero horizontal movement of the plates during the lift.
I've seen smaller metal plates (1.25kg, 2.5kg) shift on the bar during the movement if not placed inside the collars, but never the smaller rubber ones (0.5kg, 1kg, 1.5kg).