Depending on the use cases, we can send a message without expecting any return from the destination: this pattern is named event message and uses InOnlyMEP. For instance, when you read a file from the filesystem, you just process the file content, without returning anything to the endpoint that read the file. In that case, the component responsible for reading the filesystem will define an InOnlyMEP.
On the other hand, you may want to implement a request reply pattern: a response message should be returned to the sender of the request message, and so it uses an InOutMEP. For instance, you receive a SOAP Request from a WebService component, so you should return a SOAP Response (or SOAP Fault) to the message sender.