The type of photos you're taking is a huge factor in choosing the location and environment where you'll be doing the shoot. For portraits, you could use a studio, a home, a nice place outdoors, and so on. Often, clients will have ideas for the "look" they want their images to have, and it's up to you to help make that look happen. Finding the right location is a big part of that process.
Location scouting is a good way to get a feeling for where to set a photo shoot. If you know the location ahead of time, you can plan for lighting, lenses, and any additional gear you might need to make the shoot a success (see Figure 2). Choosing a known location may inspire ideas for creating images and give you a chance to pre-visualize the shot. The unique qualities of a certain spot may spark your imagination and cause you to try something special.
Figure 2 This image was photographed in a local library that I've visited several times. I knew what to expect at the location even before I arrived, so I was able to anticipate and plan my shots ahead of time.
You can't always check out every spot ahead of time, but chances are that you'll at least have an idea of what to expect from the place where you'll be shooting. I've been to photography events where I didn't have the opportunity to view the location ahead of time, and also very little time to do my actual shooting, but I've found that this lack of information can actually be beneficial in some ways. I'm forced to think and react very quickly, which helps with my speed when in a "normal" shooting environment.