I have not seen this feature in the VCO's of Doepfer but old analogue synths had it.

It would help improve FM synthesis.

Hard synchronising VCO's would be a desirable feature if the phase at which each VCO is at is continuously adjustable (1 to 179 degrees) , before locking it.

I do not have a system yet so apologies is the above post is stupid.

Regards

Anoop