Designers should have the user in mind when they make design decisions. We use safe size grids because some users have older monitors. We add alternate text becasue people with disabilities are unable to convert images into readable format. We keep in mind that not every user has the latest version of browsers, operating systems or computers. So we design with older monitors, browsers and compouters in mind.
The focus on the convenience and accessibility of the site to allow the maximum number of users to benifit from your site is known as user centered design.