Coming to the real question you have - why multiple IP addresses are needed, if at all. This is more of a practical implementation discussion. IF the UE asks to be connected to two different APNs (Access Point Name) (please note the emphasis on the If) AND IF the Service Provider has implemented the APNs on two different PGWs then the UE will be setup with two PDN Connections and each of the PGWs will give the UE a different IP address. Remember, as far as the external routers are concerned the packets are coming from a PGW and the IP addresses are at the PGW and the packets destined for the UE must be sent to the PGW. It is the PGW that knows which EPS bearer to tunnel these packets through. Does the Service Provider have to configure the APNs in two separate PGWs - NO. But, for larger networks and as an example networks who serve multiple end user clients the Service Providers are almost required through their Service Level Agreements (SLAs) to take this approach.
Hope that helps.