There is few use cases of a full screen keyboard that can be useful. For example, using it to type large amounts of text. It allows you to see more of paragraphs in one go and allows easier scrolling due to more screen estate for typed text rather then one single line.
How do I make my keyboard not full screen?
Using the F11 key on your computer's keyboard will let you both enter and exit full-screen mode in many applications. If you use a laptop, you might need to press Fn + F11 to activate this keyboard shortcut.