Easy to explain, also 3G (which actually is called WCDMA UMTS but some people prefer 3G, maybe they think it is shorter thing to to say...?) provides wireless packet connectivity of many types just like GSM, plus a handful of oter new protocols. So when you make a CS voice call with your 3G-only network, it handles the packet flow over the WCDMA air interface. Also you have the advantage of having simultaneous PS access which people notice for example when surfing with the phone while having a voice call.