Our main reason for studying bonobos is incredibly narcissistic. We want to know about ourselves.
Chimpanzees, bonobos and humans share a common evolutionary ancestor. This means around 6 million years ago, apelike creatures were running around. Some would evolve into chimps, some into bonobos, and some into us.
Because of researchers like Jane Goodall, Richard Wrangham, Christophe Boesche, John Mitani, Tetsuro Matsuswa, David Watts and Toshida Nishida, we know a whole lot about chimpanzees. The years these people, along with countless others, spent in the forest watching chimps, has given us an intimate view of how chimpanzees societies work. In some ways, these societies are so close to humans, we assumed our common ancestor was very chimpanzee like.
By this, we mean they used tools, had sophisticated cooperation, and occasionally formed war parties to hunt down, torture, and kill the enemy.
But what if they were more like bonobos? It's entirely possible that our ancestor lived in an egalitarian society that didn't cooperatively hunt at all.
And even more interestingly, if traits such as hunting and war evolved later in both humans and chimps from a bonobo-like common ancestor - then how did they evolve, and why?