Hi OP. I’m a gynaecologist.
You would need to have no underwear in for several reasons:
For any lower abdomen/pelvic surgery, the skin needs to be cleaned to reduce risk of infection when inserting strike equipment (camera and instruments)
Camera is usually through the umbilicus (but can be put in other sites) and other instruments for gynae procedures often include a a site which is at the top of the natural pubic hairline.
For general surgical keyhole (laparoscopic) surgery, instruments are only put in via your abdomen.
For gynae laparoscopic procedures, it’s standard to empty the bladder with an in/out catheter, which is to reduce the risk of bladder injury from other port site introduction, and improve the view- a really full bladder could stop the surgeon even seeing your ovary, never mind the cyst. The vulval and vaginal area will be cleaned and draped before this.
Usually, it’s then needed to move the uterus around, to see in front/behind it, to enable the attached structures of tubes and ovaries to be seen and operated on safely, and to see the exterior of the uterus itself.
This is done by inserting a speculum, then an instrument which goes into the uterine cavity, and clips onto the cervix. One of the assistants sits and after catheterising, will move the uterus around as needed at the surgeon’s direction as needed.
I hope that helps to explain.
I can’t fathom why (presume) a ward nurse told you not to wear your own underwear, gave you disposable knickers, but did not explain why, or that they would be removed.
The surgeon should also be clearly explaining the whole process before and at consent stage. I know I have seen colleagues not consider the vaginal/bladder aspect as anything but “prep”, and possibly not consider that a patient might feel violated or be concerned. Not acceptable, though.
Sorry you have had this experience, and have not had this explained clearly beforehand. You are absolutely right to question these things, which hopefully will ensure practice improves.