What legitimate law-enforcement function does the ATF have?

It makes sense to me as a tax collection agency.

I cannot think of any crimes was justify its existence, independent of the usual regulatory non-compliance issues any agency deals with.

Off the top of my head, beyond the regulatory noncompliance stuff, they investigate:

-Violations of the various federal gun laws (including related stuff like felons in possession of firearms, and armed career criminals)

-Arsons (many folks don't realize that fire/arson is one of their major areas of expertise - probably because it isn't part of the acronym - but they handle thousands of arson and arson-related cases each year)

-Bombings, attempted bombings, and illicit explosive possession and manufacture

-Gun trafficking and organized gun theft

-Alcohol and tobacco counterfeiting and trafficking

Etc.

They also partner with many other local/state/federal agencies on task forces targeting things like narcotics rings, organized crime, street gangs, and violent offenders, which invariably have a firearms nexus.

And they offer technical expert support, laboratory services, gun tracing, and training for other law enforcement agencies to aid their investigations.

I'm sure I'm forgetting some other aspects of their legitimate law enforcement function. I know we have one member who used to be an ATF agent, as well as one who used to work with the ATF on a task force, so maybe they'll chime in.