I'm Psychology and I didn't list my contributions to knowledge in my abstract. At my uni you had to keep the abstract to a page and I could only just do that anyway. I had three paragraphs, first introducing topic and methods, second main findings, third relationships between findings and main conclusions.
I had no idea what to put in the abstract and was in such a rush by then to just get it done. I don't think I ever showed it to my supervisors even! I do think spelling out how your thesis contributes to knowledge could be a good thing in an abstract if you can see a good way to do it, but I don't think it's a necessary thing IYSWIM.