If you are allergic to anesthesia they want to know. Also if they are going in to remove your appendix they want to know if you've already had it removed! 🙂
I would also think scar tissue and a slightly different anatomy would play into it. More scar tissue than the latter. Plus, i'd like to know if someone's been down there before, or if i'm their first 😀😀😀
I've had this question asked, and I've had .. several... previous surgeries. Usually the follow-up questions are "reactions to anesthesia?" "how did you heal?" "how was the pain afterward?" "any post-op problems?" and they wanted to know about scars in the area of the new surgery. I know in my case they really didn't want to cut through old scars. Also, if you're talking about abdominal surgery, they were concerned with adhesions in the abdominal cavity. Basically, they want to be prepared for any problems or abnormalities they can reasonably anticipate.
Remember, also depends if it's the anesthesiologist or surgeon asking if you've had previous surgery - reactions to meds, allergies, history of malignant hyperthermia, etc. vs. adhesions, old scars, bleeding disorders, etc.
Then there's also the part that it's part of your PMHx, and therefore something any MD (surgeon or anesthesiologist) should know before they take care of you.
Your question seems quite random; are you trying to ask something more specific?