[HKS_TRD]

I have asked myself the same question
The only thing I can think of is that it provides some form of stabilising effect and prevents the main gear from moving off centre in relation to the axles and bearings
However to do that would require it to be a tighter fit than it is and also would incite some friction when the diff is working i.e. when cornering as the spacer is not static relative to the main gear

Its the only purpose I can think of