I generally never want programs to go fullscreen because I like to keep taskbar shown, so I can keep track of time, notifications and whatnot.

Well designed video games that rely on fast and precise mouse input capture the cursor during the gameplay until menu is shown.

The only times I have to go fullscreen is for the games that fail to capture the cursor and where accidentally clicking outside of the game window leads to a loss.

Can't imagine a non-game program other than a video player that I would want fullscreen.