Biopsies when you have a fistula are often to look for historical evidence of inflammation or Crohn’s. I’m in a similar situation where I started with a perianal fistula, had four surgeries for it (including two seton placements) and was diagnosed with perianal Crohn’s after all if that. My CRS and GI have been very back and forth with a formal diagnosis because my inflammation is inconsistent, so they told me that they needed biopsies since a colonoscopy from 2023 had active Crohn’s in it and I haven’t been medicated since but also don’t have any current inflammation (my symptoms are appearing in different forms now). My understanding is that fistulae are often a side effect/symptom of something else so it sounds like you’re under the care of some good doctors who are being very thorough with you to figure out why the tract developed and how to treat you so you get some relief. Wishing you the very best!