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Researching any prospective employer serves two purposes. First it allows you to evaluate whether or not you want to work there. A potential employer often sounds promising but after researching them you might find out that it is not a good fit with your interests and professional goals. Research thus prevents you from making a mistake. Not all job offers are worth the trouble, but it's up to you to go to the trouble of finding out if it is.
Secondly, researching a company helps you to impress the interviewer. The interview process is your chance to sell yourself. Knowing as much about the company as possible shows that you are interested in the position and will be motivated to work there. You need to portray politely and positively that you are knowledgeable about the organisation and do have something to offer it. Your CV/resume may well have shown examples of your skills as a team player, but now you need to convince them that you fit their team. Research enables you to form a picture of what their "team" looks like. Solid research will thus enable you to confidently say at the end of your interview "I'm really interested in pursuing this opportunity. What's the next step?" and mean it.
When you're researching a prospective employer, irrespective of size you need to identify the areas that concern or interest you the most. These facts should come in handy when it comes time to formulate your questions. There are a myriad of facts and factors you can find out about, especially for larger companies. You only need delve as far as you need to until you feel you have a good grasp of what a particular employer is about.