To understand Why use magnets and why magnets work we need to know why they are magnets.
Current understanding is that when certain metals have more electrons spinning in one direction they influence more electrons to spin in the same direction. The cumulative force then makes a stronger magnet.
When a magnet and a copper wire move past each other the magnetic force moves electrons from one atom to another thus creating an electric potential that we use as electricity.
The human body is not made of copper but is mostly water so what effect will the magnet have on the water?
Russian scientists were trying to solve the problem of calcium depositing on the walls of steam generators ; the solution they found was that water passed over a strong magnet could hold the calcium in solution longer so that less was deposited in the boiler.
The answer seems to be that water made up of 2 hydrogen and one oxygen has the 2 hydrogen on one side of the oxygen, this makes it positive on one side and negative on the other, this makes water stick together in long chains like spaghetti. When the electrons are sped up by the magnet the hydrogen appears to move further apart on the oxygen so that the water sticks together in hexagons more like spaghetti-Os.
To complete the illustration imagine yourself a thirsty cell and all the water is like a pile of spaghetti that you can't get your mouth around.
Along comes some water that is like the spaghetti-Os that slide right in and out and carry more sauce because of their shape. Now your cells can drink and eat.
I hope this simplified illustration helps you understand Why use magnets ans what is expected from magnet therapy. It dose not treat or cure disease; it is intended to help the body deal with a problem, a symptom, water not moving through the tissue and doing its work.
So under how to use magnet therapy we can discuss how to get magnet energized water into the problem area so that the natural processes of your body can solve the problem causing the pain.
As you consider the how, Why use magnets will be easier to understand.