Good width and depth provides the player on the ball with all around support so that there are options to play the ball forward, square or back. The more options a player has, the less likely they will lose the ball. Creating depth means spacing out up and down the field. You're attempting to pull and draw defenders out of position. This allows us to play through the lines with a pass or create space for the player on the ball to progress via carry or pass.
Creating width means spacing out side to side on the field. Good width provides opportunities to attack on either side of the field and up the middle of the field. This creates space horizontally - think wingers hugging the touchline, this pulls defending full-backs away from their central defenders creating space in-between the channels. This allows for attacking midfielders to move into the channels - half-space.