The thing you're getting by switching is the benefit of the information provided by the person who revealed an empty door.

Before a door is open, you have a 1/3 chance of selecting correctly.

After you select a door, the host picks from the other two doors. This provides extra information you didn't have during your initial selection. The host points to a door they know is a dud and asks for it to open. So now you're left with the question "Did I pick the correct door on the first go? Or did the host skip the door that had the prize?" There's a 1/3 chance you picked the right door initially and a 2/3 chance the host had to avoid the prize-door.