Why is Hannibal involved in the case at all? Spoilers

Actually they weren't that far apart. Buffalo Bill is found in Ohio, while Lecter worked out of Baltimore, Maryland. Gumb's first victim was from West Virginia, not far from Maryland.

But even not knowing that, Crawford knew that Lecter was a psychiatrist, and still had some interest in the practice, so thought he would be helpful, given the right incentive.

If my memory is correct, I belive the viewer is given certain bits of information:

Behavior/psychological forensics is a fairly new branch of the FBI. Crawford (?) is leading the development of this field, and therefor uses a questionaire to get info from ALL incarcerated serial killers at the time - Lecter is the most "interesting" serial killer, but he refuses to participate.

Being busy with the ongoing Buffalo Bill case, Crawford sends student Clarice, hoping she'll be able to get through to Lecter. This might be useful in the Bill case, in the sense that all scientific findings might be useful to the field.

However, Lecter secretly already knows who Buffalo Bill is, and it is infact him who quickly turns the first conversation on to Bill.

Being the only agent that has been able to get any useful information out of Lecter, Clarice is kept on the case - even more so when the FBI/Crawford realizes Lecter has information about the killers identity.

The questionaire was never an excuse to question Lecter about Bill - it just so happened that Lecter had info about an ongoing case he was willing to share.

This is not a plot hole or weak writing - this is infact the main plot point of the film.

Lector knew Buffalo Bill through Benjamin Raspail

Just a clarification: Hannibal Lector's patient records were destroyed by court order After he was captured in order to maintain the confidentiality of his patients. He didn't destroy them himself.

The characters borrow traits from real life killers. Bill using Bundy's plaster cast to attack women, the skin suit from Ed Gein... Hannibal helping the FBI catch Bill is based on Bundy trying to help the bureau catch the green river killer.

They wanted him to build the profile because he is both a psychiatrist and a psychopath. It had nothing to do with knowing him or not.

This movie follows Manhunter / Red Dragon

Manhunter / Red Dragon reveals that Lecter is an extraordinarily good forensic psychologist. For years, he had been working with the FBI, including Agent Crawford and Agent Graham (Edward Norton), to crack many hard cases. In Manhunter / Red Dragon, Crawford (who was Agent Graham’s supervisor) nudges Graham to seek Lecter’s help to solve a case, just as crawford nudges Clarisse to seek Lector’s help to solve a case in this movie

The point is that the events in Manhunter / Red Dragon explain why Crawford would believe Lecter could be useful in helping them catch Buffalo Bill. Lecter had a long history of solving cases by catching stuff that everyone else misses

SCRIPT.