That's a really charitable way of saying "handle petty stuff that the FBI can't justify spending resources on".
Every specialty police department exists for this reason and this reason only. Because the mission is often so petty they wouldn't get any resources if it was obvious that resource allocation to that task was resource allocation away from other policing.
When you have real problems the real police have no problems allocating resources and whipping up dedicated teams. When you feel like using state violence to harass drunken college kids you create a campus PD. When you feel like using state violence to kick the homeless out of the train station you create transit cops. Etc. etc. Even if you're an investigative agency you will have no problem getting the local cops to provide muscle if your needs are legitimate. "Look at us working with the <pick three letters>" is the kind of photo op local departments love, so long as it reflects well on them.