On one side you have high pressure air coming from the source, which could be an air compressor or air tank. On the other end of the equation is the destination, which could be a tire or an air based device that you are using. This is no different than power supply flowing from the mains to the various electronic devices in your home. Just like how the power supply can sometimes fluctuate and hence you use a surge protector, you have this air regulator to maintain consistent air flow.
That is just what an air regulator does. How does this help you? As it turns out, it helps in some subtle and very obvious ways.
Irrespective of what kind of air pressure application you have in store for your air compressor, all of your air based tools would prefer a consistent supply of air. If the supply of air was constantly fluctuating, then your air based devices will have to adjust to the changing flow, increasing wear and tear exponentially. You will find all of your air based devices going through more repairs, maintenance and eventual replacement.
A particular application of high pressure air, spray paint is not possible at all without a consistent flow of air. You could have the best air compressor in town and you may be the most gifted painter, but without an air regulator there will always be times when the pressure just cuts off. Painting is a process of skill and concentration and you can’t let something like air pressure come between you and your work.
In most cases, the regulator would have been included with your compressor. If your main application is spray painting, we would insist on a separately purchased regulator. The only thing that you need to check is how easy and convenient it is to change the pressure.