What happens if a new client comes to you with a website they want redesigned, but because analytics show that very few people visit or convert from mobile on the website, they want you to prioritize for desktop? Your data tells you this is wrong, but theirs suggests otherwise.
As a web designer, you may be starting with little to no data of your own. It’s not until a website has launched (or relaunched) that you can start gathering real analytics on its performance. Even then, it can take awhile to draw any meaningful insights from it.
So, in many cases, what web designers are working with before and while they design a new website is industry data: audience insights; competitive research; keyword analysis; Google announcements; marketing surveys and reports.