A web designer is where your website ideas are born, created, and then summarized into a form called wire frame where they can then proceed to prototyping.
Mind you, it is not a functional website, but think of it as a picture of the website that you want to create. The menus are not really interactive at this stage, and everything is used as a reference.
This process involves the client and the designer to conceptualize the idea of the website, taking a gradually and transparent approach in bringing the ideal website to life.
The web designer will employ the help of tools like Adobe Photoshop to aid in the creative process.
The designer will often consider all things visual for the website.
They will also consider things like Balance, Contrast, Emphasis, Consistency, and Unity of the entire website.
Balance will involve the use of the different elements, like colors, for example, on the website.
Not only that, but the correct usage and application of these elements are favoured, considered and decided over a process by the designer.
Contrast is how certain colors and placement of the website elements that will draw the visitor's attention.
This is also important to find out how various elements can also complement each other to provide a better visual.
Emphasis will be the process where important information of the website and its trade, and how these information will be brought the the attention of the visitors to the website.
Remember those school days where you think everything in the textbook is important for the examinations and then you will proceed to highlight everything?
Same principles here. If you highlight every aspect of your website, the attention that you are trying to draw to a certain piece of information that you were trying to highlight will be diluted.
To get a uniformed approach for the website, web designers will also consider if your highlights, and messages on your website are consistent from page to page.
This is also where website designers will involve the client and let them understand the principles of User Interface and User eXperience (UI/UX). This is now a critical factor in how your website will flow.
This is largely based on the human brain's capacity, using the Gestalt theory, and how the brains process various information and categorising them, etc.