What is the purpose of a rear diffuser?

A rear diffuser efficiently creates an increase downforce through a vacuum created by low pressure under the vehicle and high pressure behind the vehicle.

Diffuser in a car Download Scientific Diagram

M Performance Carbon Fiber Rear Diffuser w/ Brake Light - BMW F30

Carbon Fiber Rear Diffuser with LED Brake Light - BMW F30 M Sport 335i

Diffuser

BMW 3 Series F30 F31 with Twin Exit Carbon Effect Rear Diffuser

Diffuser (automotive) - Wikipedia

Rear Diffuser For Mazda MX-5 NC

Diffusers, Engineering basics

Get Diffused: What is a Diffuser and what does it do

Universal Rear Diffusers - Carbon Fibre

AVR Jaguar F-Pace Rear Diffuser(V2) Fits All 2.0 LTR Petrol And Diesel

Diffusers, Engineering basics