EMC open area testing sites

Hi...we are programming to build an EMC open area testing site in our lab.. need to understand what is the function of the ground plane ? do we really need it ?
thnx

I would be more worried about the benches and test equipment reflecting stuff!
the ground plane is needed for immunity testing, as it is part of the specification, I believe. but for other testing, it might be more trouble to have it than not.