Researching the company you are applying for will help you decide if the work the company does appeals to you. You may end up researching the companies at different stages of the job hunt. Before applying, before the interview, after the interview/before accepting a position. Some information you may want to look at.
Showing up to your interview well informed about your potential employer is an important way to show your enthusiasm about working for the firm. You should come to the interview prepared with a working knowledge of the company, its mission, and specialties. Your knowledge about the employer will show that you are selective about the jobs you apply for and that you actually find the company's work interesting. It will also demonstrate that you are thorough, detail-oriented, and professional--all favorable qualities when you are representing the organization as an employee.