Protect the T from having to deal with their "work/job" constantly, thus protecting their sanity. And protect them from any violations of boundaries and/or ethics, thus protecting their reputation.

It protects the clients psych. Most post-therapy relationships that you hear/read about have negative outcomes. Very rarely do they work out. So avoiding a relationship, other than the therapeutic one, protects the client from another bad relationship and more hurts.

I had an eye-opening experience about 2-3 weeks ago in session. I have symptoms of Asperger's so my T wanted to just have a conversation for the therapy hour. This was an exercise to build my social skills. It was so awkward to make that switch of T being like a "friend" instead of T. It took at least 10-15 minutes for me to shift out of therapy mode and into RL mode lol. Still during the course of the conversation, there were times it got really awkward. Not something I'd want to experience again (in or outside of therapy). I love her as my T, but that's where it stays. If we were friends outside of therapy, then I'd lose a perfectly good T who knows exactly what I need at exactly the right times. I'd hate to lose that. I can go to her about almost anything and know she can offer me help.