There is a broad debate regarding the function of play behaviour in mammals. Generally, there are two hypotheses. First, play is effectively training — the development of motor skills — for aggressive fights later in life. Second, play is socially functional in its own right by cementing an animal's social rank within a group. These are not necessarily mutually exclusive, although they are often portrayed as such in the literature.
Macropods (such as kangaroos and wallabies) are classic examples of animals that exhibit play fighting. In both kangaroos and wallabies, adult males tend to compete with other males for access to females through aggressive fights, so it's perhaps intuitive why young males might play fight to develop their fighting skills. However, it is currently unknown why juveniles and females engage in play fighting, and whether play fighting is influential in determining an individual's current social rank.
There are a variety of potential projects, including the study of how play behaviour develops in juveniles and subadults, how play is related to the sex of individuals and the social rank of those individuals, or the function of play in the maintenance of social networks and how those networks might change over time.