Why is it important to know who will be visiting your Web site?
Because, as Jesse James Garrett said, "in virtually ever case, a Web site is a self service product" (Garrett, Elements of User Experience, 10).
What does that mean for you? You need to understand your users and what they'll want to be able to do once they arrive at your Web site.
Let me put this as bluntly as possible:
If your users don't understand your Web site, your Web site will be a failure.
EVERY step of your planning process should include a consideration of how your user is going to be interacting with the system, what their own goal is going to be, and how they'll get there without getting frustrated or confused.
You may also find it useful to create a few sample personas, meaning you create fictional user profiles that represent types of regular visitors to your site. The idea there is that you will keep these personas in mind when you make decisions about how to build the site.