Human beings are indeed primates. If you wish to study the development of humans, then primates should be integral in this action. It is believed that primates appeared approximately 60 million years ago and that the development of modern Humans (Homo Sapiens Sapiens) started about 5 – 6 million years ago. Ostensibly we have shared ancestors for approximately 50 million years. We share many common behavioral characteristics too. We are social beings, we use tools to complete certain tasks, and have the ability to communicate with each other.
Studying non-human primates may help us understand how we developed. For example, why do we walk on two legs? When did we become bipedal and what circumstances allowed this trait to develop? Most existing primates are quadripedal (move on 4 limbs), but use bipedalism for certain tasks (gathering food for example). By looking at those species of nonhuman primates that are sometimes bipedal, and discovering the circumstances in which they display this trait, we may gain some understanding of the factors that promoted the evolution of bipedality in humans.
Humans and primates (non-human) share physiological characteristics. Our brain is organised in a similar way to primates. For example, the process of vision and the role the brain plays in this process. If we can study this process in primates it helps understand how the process works in humans and can aid in the treatment of any ocular problems. Study of primates has extended to reproduction, the spread of disease (such as AIDS) and viruses and to their treatment--contributing to improved health outcomes for humans and primates.
In a social sense we can study interactions of primates from different species and theorize about how surroundings and environment affect social behaviour. For example, the Male Titi monkeys appear to exhibit behaviour that resembles jealousy, but male Squirrel monkeys do not. Why?