Initially, we do not know the other person and we go to therapy to learn about ourselves. So, if the other person discloses about themselves at that time, the "space" for us to work in gets smaller because we instantly have opinions and feelings about the other person that can get in our way of talking about us.

If I am married and my therapist tells me she is in a lesbian relationship, that is going to color my talking about my marriage. True, my own head is going to color talking about my marriage; I can imagine my therapist seems married or a lesbian but if I don't know that information, what I imagine also is about me and is very clearly about me because my therapist has not confirmed or denied that information so it all has to be coming from me. That I would imagine her married, when she is a lesbian, "adds to" my therapy, which is about me. If she just tells me she's lesbian, I'm not going to imagine so my unconscious and what's going on with me will be "stunted".

Yes, after we get in a working relationship and discuss ourselves a bit then some of our background and where we're coming is "out there" and the imagination portion is not needed anymore.

I spent 7+ years imagining my T wasn't very good with computers and electronics, wasn't "mechanical", something I am very good at and I felt kind of condescending toward her. At some point in year seven or eight I commented on that and we discussed it; it turns out she's actually very good and it is just too bad we did not know to discuss my assumptions earlier. But as we go along in therapy we learn to bring up our assumptions and that's a wonderful lesson (and just bringing them up, "I feel like you are not very mechanical") is the important part, not whether T is or is not. But if we had started with T saying, "I'm usually very good with mechanical things" I would try to "agree" with that since it is her talking about herself and that would dampen how I observe and jump to conclusions, something I wanted very much to learn not to do.