Because it ensures a page is useable on a mobile phone, even if the author didn’t take mobile into account: the screen can accommodate for about as much informa

Because it ensures a page is useable on a mobile phone, even if the author didn’t take mobile into account: the screen can accommodate for about as much information as a regular 320px computer screen. If you try to use every pixel as you would on a regular screen it would be illegible for most users.