Knowing something about the company enables you to anticipate questions and prepare your answers ahead of time. Sometimes, it pays to do some preliminary company research before you send your resume. No sense wasting your time and money on a job that doesn't fit. This tells you where the company is, what it does and how big it is. Human resource personnel need to learn about corporate structure.
Marketing managers look for ad campaigns, positioning strategy and consumer attitudes.
Accounting people need numbers. You need a general industry outlook in addition to information about your target company. Start you research as soon as your interview is scheduled. It's hard to predict how much work it will take to find the information you need until you begin looking for it. Ask your librarian for help.