Websites created for a desktop browser usually fall apart when they are opened using devices with smaller screen size. And mobile website browsers support limited design format only. So it's vital to optimize your website for it to function appropriately on all screen resolutions.
If a website designed for a desktop is opened on any other device it requires a lot of zooming in, scrolling and thus it is difficult to navigate. If you want to deliver the best user experience to your online visitors then you have to optimize your website for different screen resolutions.