An interview is simply a tool used by a potential employer to assess a candidate's ability to perform a role. The interview will normally be the first time that the employer has the opportunity to meet you. They need to assess whether or not you have the qualities to perform the role competently, the experience that you have so far in a similar role, and also whether they like you as a person and whether you are likely to fit into the team environment. It is also your chance to find out if you want to work for the company, so it is a two-way process.
There are different kinds of interviews though, and some jobs will require more than one, and there may be more than one interview, so it's just as well to be prepared for this eventuality.