If the furniture in Rex's apartment was an illusion, including the sofa, how were he and Phoebe able to sit on it? And why not just have a real apartment? It wouldn't have been that difficult, and Rex has to be living somewhere.
He could have just made Phoebe think she was sitting on a sofa. They would really have been sitting on the floor, but the magic would make Phoebe think otherwise - since he made Prue not even remember that she had the tiara. And showing Phoebe his real place of living would have given the sisters an opportunity to possibly know where to find him or surprise him, meaning he wouldn't have as much control over their movements.
And maybe he also wanted to test out how strong the magic was? If it doesn't work, he could easily make up any story he wants about why they're in an empty room. If it does, then he knows it's strong enough to complete the next part of the plan.
