Hiding in Plain Sight
Calling in the FBI is usually a last-ditch effort by the local police. FBI agents will be put on stand-by and will "hide in plain sight." For situations that are dicey, their skills will be necessary, so they need to be close by in order to make sure everything goes smoothly. A lot of the time, they will set up a home base near the situation.
Helping All Over
The FBI is always there to help. Whenever a federal, state, or local law enforcement group needs help investigating or de-escalating a situation, the FBI will be there. They're also known for flying out agents to help ally countries.