Crouching is a useful way to expand your movement options. By crouching, you can actually walk a little bit off the edge of a block, and you can’t fall. This is ideal when you’re building a bridge-like structure.
Be careful you don’t let go of the crouch button. If you do, then your character will fall. It’s especially dangerous to try and use this trick while you’re high in the air, but you take your life in your own hands.