ELI5: Air is heavy and it takes alot of work to push it out of the way. By having someone else push the air out of the way, you can save that energy and increase your MPG.

Most cars are aerodynamic enough that slip streaming isnt something you can do anymore as the air returns back to where it was faster than you can hope to occupy that space. If you had 2 big boxy vehicles, than you could slipstream one off of the other and decrease total fuel usage at the cost of an increased accident risk as any gap will demish the benefits.