You can actually drag windows to one of these desktops, if you want, then switch to the window by clicking it.With multiple desktops you can organize your workflow, allowing you to do things like research on one desktop while you write on another. And you can add as many desktops as you like by clicking the “+” button at far right.
To switch between desktops, you could just open Mission Control then click the desktop you want to open. It’s a lot faster, however, to use the keyboard shortcuts Control+Right and Control+Left, or to swipe three fingers to the left or the right. These will both switch your desktops immediately, and are a nice compliment to the keyboard and mouse shortcuts I mentioned earlier.
If you want a particular application to always show up on a certain desktop, or even on all desktops, simply right-click its dock icon, then move to the Options submenu.