Why is Hannibal involved in the case at all? Spoilers

In the film it is stated before he was captured Hannibal destroyed all his client records.

When Crawford send Starling under the false pretense to gather data through a questionnaire. He really wants Hannibals help on the Buffalo Bill case as he feels he will open up to Starling.

He would have no idea the two killers would have met.

Buffalo Bill is killing in Texas and the lower mid west. Hannibal is jailed and was a practioner in Baltimore. It's such a leap that Hannibal would be any help. He hardly has respect for Hannibal as the fake reward offered would mean Hannibal would not help in the future.

Overall it's all luck and a weak plot point commonly overlooked.

I can understand Hannibal may have the capacity to help as a psychiatrist and killer himself. But it's such a gamble.