On straight putts, aiming at the middle of the hole maximizes your margin for error. If you slightly pull or push the ball, there’s a chance it may still slide in the edge. On breaking putts, the ball is coming into the hole from a different angle so the centre of the hole (on the direction the ball is travelling when it reaches the hole) is in a different position.
It’s important you understand this distinction as trying to drop the ball in the centre of the hole as you look at it will mean you’ll often miss on the low side if the pace isn’t spot on.
By creating a gate with two tees, it’s easy to visualize where the centre of the hole is and allow the ball to roll down the fall line between the tees and into the ‘centre’ of the cup.