The main purpose is to gain better control over the flow and a good side effect is less over spray thus getting the paint to last longer.
Use a thick drinking straw about the size of the nozzle, cut it to get a shorter bit.
Put the straw over the nozzle, hold the end down a suiteable container and spray away. The paint you get out in the container needs to gas out for a while before you put on a lid. I usually use a film container with a little hole in the lid and leaves it over night to gas out properly before putting on a whole lid.
Now you you can use it as is in your airbrush or maybe thin i a little more, normally not neccesary it's fine as is.
Good luck!
Niclas