A client calling a normal wcf void function (request/response) has to wait for the server to finish the call. So the client blocks until the server finishes executing the code and sends back HTTP 200 Status code (OK) to the client.
A OneWay operation gets finished as soon as the server receives the message and sends an acknowledgement (HTTP 202). The client does not have to wait for the call on the server to finish.
This makes OneWay operations more scalable as compared to normal operations (even if they are called asynhronously)