how would this work for bigger monitors?

Later on in another css course if you are following the front end track (I think either css layout basics or responsive layouts), you will start to see why you should develop websites from a mobile first approach.

However to answer your question, in narrower screen sizes such as tablets and phones you will want to take up as much of the browser width as possible as content will be too narrow and cramped when viewing them on these kind of devices. A phone layout will typically display content in a one column layout due to the narrow screen in portrait mode. A tablet typically shows content in a two column layout, and a wider desktop would typically display content in a three plus column layout due to the screen width being more open you can therefore use more of the width. However on larger screen sizes past this break point specified in your question the reason why the content reduces to a smaller width is because on these larger screen resolutions you dont want the content displaying too wide because that is just too difficult to read. Check other webpages out and inspect their container/wrapper with the chrome developer tools and you will see that they would have styled it to be a smaller width for larger screen sizes to prevent content from becoming too wide.

Sorry for the long clarification, I hope it helps you understand in some way.