It’s a serious question and one you need to ask yourself before you start working with a developer or designer. Why? The answer can’t be because the latest shiny business book told you that you needed one or that your cousin the Social Media expert said you are missing out. You have to have a reason that you believe in.
The process of building a website is going to be frustrating unless you are building it yourself. (Even then, it’s not going to be nearly as easy as you think) If you aren’t 100% in then it will be easy to either give up or just let your designer or developer run the show. Both cases are bad for your project.
Answer the question. Write it down so you don’t forget it. Review it from time to time to remind yourself why you are going through all of this. You are the vision keeper of your website project. If you lose sight of the end-goal, your project will never make it.