Being successful by using certain sophisticated techniques each decade depending on what type of crime is being committed the most at that time.
Sometimes even spying on the American citizens so they can keep up with every threat that is made in the United States. (like when they stopped a terrorist attack on December 31st, 2015)
The Bureau has multiple departments in which each member are required to do numerous unpaid collateral duties. -being on special SWAT units
Doing tactical training on a regular basis to be prepared for many facets of crime.
The FBI enter cases only when they've already gathered articular facts, suspects have specific allegations against them, and relevant information is available.
FBI agents don't give information to the public because they don't want to mess up the case if it goes to trial, and doesn't want the media the sway citizens into their perspective. Not wanting pester with their investigation.
The agency is important to our nation because they do their best to protect citizens rights in the Constitution, while also providing a check in balance to the government.