The reason for such focused degrees allows the agents to be able to apprehend offenders within those fields; or they identify other potential crimes that the offender committed. For example: Al Capone evaded taxes and possibly had many individuals killed, but they lacked evidence on those killings. So, he was apprehended for the crimes he did commit and was found guilty of tax evasion. That was how they justified locking him up.
They do have agents that are basically law enforcement officers, but all agents have degrees, preferably master's and Ph.D in the fields they excel at or worked in. I hope that helps you map out your educational goals and future in the FBI. Best of luck.