Next to GRE scores and course grades, it is the most important part of your application. Put time and effort into it. It provides the program with a sense of who you are and what you are interested in. It also provides a sample of your writing ability and your ability to put interesting thoughts coherently on paper. The first stage in the process is to exclude applicants with GRE's below a certain cut-off.
Some programs bypass the interview process and select candidates based on their application.
However, many programs conduct interviews. Between 20 and 40 applicants are invited for interviews at the institution. In selecting applications for interview, all of their application materials are considered. Once a student makes it to the interview, the impression they make at the interview is critical to the final decision on their application. Applicants are invited for interviews between February and early April.