Why is it best to leave an empty slot between two expansion cards?

Answer 1

One reason is to allow cooling air to get between the cards. Many people who think they know computers have used every expansion card slot they have and cooked the computer
To help prevent them from overheating.

Answer 2

Because of cooling between one slot to another slot

Answer 3

To leave space for the cards to breathe. If they are too close to each-other they can become overheated and will eventually fail.

Answer 4

So the card can breath and cool over rather than over head and sending a chain reaction through all the expansion cards.