Please first read through
Next, in general, you may wish to do two kinds of test
1. Study app performance when there are other devices on the network
2. Study app performance when there other apps running on the same device (communicating with their respective servers)
Now note that the virtual nodes in NetSim (simulator) generate background traffic by interacting with one/many virtual servers but will not interact with the real server.
Therefore you may need a few real clients for testing some application specific stuff that may require multiple real clients i.e multicast traffic
Next, let's say you want to run test 2, but for various reasons, you can't have multiple apps running on the real client. Then the real client will handle your app while the mapped virtual client can be used for running the traffic of the other applications. Since, both your app and the other apps/traffic in the virtual client use the same network/links, the performance your app may degrade.