You know what content you want on the website but have no clue how to present it to the user.
You know what content you want on the website, and you have the layout in your mind, but you don’t know how to implement it.
In both cases you will need to explain your thoughts to the web designer. Although most people who read those lines are probably thinking that being in the second situation is better then being in the first situation. However, real life experience shows the opposite to be true. Giving a web designer the complete freedom of action regarding the web design based solely on the website content is usually a smart thing to do. You will find that explaining to the web designer what the nature of your website is, whether it’s a product that you want to sell or a hobby item, is much easier then trying to explain to him the temperate of the color schema or an undefined shape that you would like to have in the website header.