So the fact that some (or was it many) airports with VFR towers have radar approach control is submerged somewhere in the computation of benefit, but it is not possible to see what the effect of radar is explicitly.
The APP RDR will sequence IFR a/c to the duty runway for the [former] VFR/ [now] Class D TWR. In the handoff, if the TWR has a radar display, the local controller can see the position and speed of the inbound IFR, thus aiding the sorting of the runway sequence with local traffic. YSBK [as an example] are capable of doing a similar thing.
Hope that helps.
G'day
PS This was the intention for places like MC, AY, CH, etc., but either the radar picture won't go that low or AsA was unwilling. It was never envisaged that our Class D towers would control the volume of airspace they do. The intention was that the Centre would sequence and pickup outside a basic aerodrome CTR.