It might be a company wants to keep a uniform feel to it web pages using the company colors or logos. This is why a lot of times they will set up custom style sheets to manage these. If they need to make a change they just change the CSS and not every web page if it was embedded in the HTML of every page.
Good design will also involve making the sites responsive. This mean grabbing the OS, browser and device settings then custom building the page to the device and the devices screen size.