This is an important question to ask for a couple of reasons. First of all, it's not always clear from a company Web site or job fair display what exactly it's like to work for the company. Modern business culture runs the gamut from suits and ties to shorts and flip-flops. Some companies expect all employees to clock in at 9 a.m. and attend regularly scheduled meetings, while others encourage working from home and gathering via Web conference.
By asking about the company culture, you'll get a better picture if this is indeed the kind of environment in which you'd like to work. If workplace culture isn't an issue -- you'll take any job, period -- then the question is more of a lead in to show how you and your personal style would fit into that culture. If you've always felt that dressing up for work sets a professional standard, here's a great opportunity to share your thoughts. If you think you do your best work in a beanbag chair, make that known as well.