As a website owner or web designer you have to take into consideration what are the most common screen resolutions for the intended target audience
of the website to determine what web page width is suitable for the website.
Have you visited a website that has a web page width too wide for your monitor or browser window? You have to use the horizontal scroll to go back and forth reading and/or looking for information on the page. This makes reading the website very hard and can be very annoying if the website has some information the visitor is interested in.