Not everyone who abuses animals becomes a serial killer, but people who do become serial killers and commit other violent crimes do have a statistically significant history of animal abuse in their backgrounds. Tracking animal abuse creates a database from which more information can be garnered. It is a crime in itself as well.
The FBI is making good use of its time by tracking animal abuse, as it would homicide, arson, and assault. These are crimes against defenseless creatures, making it worthwhile. Also, people that abuse animals may turn to more violent crime. It has been shown that psychopaths tortured animals as youth, making it particularly important to prevent future crime.
I don't think that tracking animal abuse is a waste of time. Studies have shown that many people who abuse animals go on to abuse and murder humans. If we can stop these heartless people early, then maybe we can keep them from committing further crimes against both animals and humans.
The FBI is not wasting its time by treating crimes against animals as seriously as those involving homicide, arson, and assault, because abusing innocent animals is just as wrong as abusing people or property. Also, people who commit these types of crimes generally have violent tendencies, and for some of them, it's only a matter of time before they graduate from harming animals to harming people.