I believe there are two main reasons for the "investigation"...
One, by stating there is an ongoing investigation FBI officials now have cover to deflect *any* questions even remotely related to the Bidens, Ukraine, anything. "Sorry, we cannot comment on an ongoing investigation..." Done. That cover can also be somewhat extended to anyone and everyone connected. "Sorry, cannot comment on that..."
Two, they do have a few trusted (read that as corrupt) upper level people investigating, maybe even including using low level agents who will dutifully do what they are told. What they're really looking for is anything damning to the Bidens and peripherally obama and the Clintons. Once they know what evidence is out there they can turn it over to "those who must be protected" so they can work to counter it, buy it off, or the "trusted" FBI agents might even help them quietly destroy it or lock it away...
Nope, the race is on right now, and the good guys (that's us, those that oppose the Bidens and their ilk) are losing.