I agree with larrygates, by gaining an understanding of both politics and government we are able to make certain that individuals are able to make informed decisions about who they are putting in office.
I assume by the question you mean why is the study of politics necessary. If one reads the newspaper or listens to the evening news; one will notice how the reader/listener is carefully guided through the procedures of government (e.g. sixty votes are required to close debate in the U.S. Senate.) At the same time, if one turns to the Sports page, it is assumed one knows all the rules, scoring methods, etc. This is a sad commentary on the knowledge of the workings of our government by those whom it serves. It is essential that all citizens be informed citizens; that they enter the ballot booth knowing the issues, the positions of all the candidates, and the consequences of voting for a particular candidate. I can think of no better illustration than my own state where an accused unemployed felon won a U.S. Senate nomination over a more experienced candidate simply because his name appeared first on the ballot. The end result is that the candidate for the opposing party won handily. By learning politics one can become an informed voter.
The meaning of this question is not very clear. Are you asking what professions a person might engage in if they study politics in college? Or are you asking what benefit we get from studying politics?
If we study politics, we benefit because we come to learn something about how people make decisions that affect the welfare of a group that they are part of. We learn about what sorts of things people can do to try to gain power in their group and we learn about what sorts of things they are likely to do once they gain that power.
Once we understand this, we know a bit more about the way the world works. We are better able to understand human behavior and we are better able to predict what will happen in given circumstances where politics is involved.