In the bad old days (i.e., the day before yesterday), when you made an app fullscreen on your Mac, the fullscreen app got added to a space in Mission Control at the far right. That was fine for users who never had more than one desktop space in Mission Control (that is, just about everyone). However, I usually have six or seven desktop spaces in play at any time, so if I were working on something on, say, my first desktop, and had to get to the fullscreen space, I would have to navigate through all the other spaces via the keyboard (Control + ← or Control + →), or show the Mission Control bar and mouse over to it. Like a savage.
El Capitan, however, allows you to make and position a fullscreen app beside whichever Mission Control desktop you like: just grab the app’s window and slam it against the top of the screen, then drag its thumbnail beside the desktop space you want it to neighbor.
Here’s a video that shows how it works.