Disguising has three main uses if you plan to die.
The most common use of disguising and then dying is to die as a Town Investigative and exonerate your mafia members. For example I say I'm an Investigator and my Godfather is a Medium, so anyone who says I don't fit is clearly mafia. The next night we kill a spy and I disguise as the Investigator. I put down in my will "N1 (My Godfather) Medium, N2 (some dead guy) (dead guy's role), N3 (the Investigator) Framer" and I get lynched Day 4.
Another use of dying while disguised is taking up claimspace. So you disguise as a Bodyguard, get lynched, and hope to get the real one lynched as an Arsonist. Hope your Godfather didn't claim BG.
The third common use is screwing up Sheriffs. Disguise as anyone who claimed and wait for them bust you. When you get lynched, hopefully the Sheriff is executed as NE. Of course some Jailors ignore non witch NE in an SK game instead of executing them, which screws with your "make jailor lose his executions" tactic, but if you're stuck with one of those Jailors after you screw with a Sheriff, at least you'll have an executioner vote with you so it's not a total loss.
In custom games, the options to Disguise as widens, but the other options are a bit dubious. If you die in one of the Any slots, you take away claimspace from your allies.
And that brings me to a question, aside from screwing up Sheriffs is there any reason to disguise as a Vampire if you can disguise as a Doctor?
Suppose you have a list like 7 Town, 3 Mafia, 1 NE, 1 random neutral, and 3 Any.
Here is an example: Jailor, Sheriff, Lookout, Mayor, Town Protective, Town Support, Town Investigative, Godfather, Random Mafia, Mafia Deception, Random Neutral, Neutral Evil, Any, Any, Any.
You know player X is a Vampire by (insert what he said that made him obvious Vampire even though you have no cccconsig). You believe you figured out someone who is a Doctor, so your disguise options are "guy I'm sure is a Vampire," "guy who claimed Doctor and I'm sure is Doctor," and "someone with no claim." Is there any reason besides screwing with Sheriffs to disguise as a Vampire? You could imply by your actions prior to dying being either the original or a converted one, but I still see no point.