They want to take the other variables out of the equation and test each with two different speeds of networks. Without an isolation you wouldn't be able to test

They want to take the other variables out of the equation and test each with two different speeds of networks. Without an isolation you wouldn't be able to test the speed of the device with any meaningful accuracy. (One test could have happened when the wifi was interfering with another network that was streaming a bunch of video while another test could have been while she office was at lunch. I've had troublesome network interference when the microwave or a wireless phone was in use. Source: did network installs for clients when wifi was brand new and harder to setup reliably)