It was done to get more cold oil to the rocker arms sooner to reduce valve noise during warm up.

On the low pressure 235 engines the rocker arm oiling was from oil taken from the rear cam bearing (which came from the rear main bearing) and piped to the head casting and (restricted) to the rocker arms. Dippper oil still from the low side of the distributor valve.
If the oil would have been taken from the high pressure side all the flow would go to the point of least resistance which would be the oil supply pipe for the dippers - which did not require high oil pressure.