On the extreme end, consider someone on a spotty and slow mobile connection going through Google results to find what they want. They're only going to give you

On the extreme end, consider someone on a spotty and slow mobile connection going through Google results to find what they want. They're only going to give you a few seconds before they hit "back" on your site and try another. The round trip for the browser to fetch the CSS file after the HTML file arrives can be enough to cause that delay.