I love being able to demo hairstyles - I only wish I could in RL!
Demos give you the chance to "try before you buy" - usually it will be the same hair that you can purchase, but with an extra prim attached advertising it as a demo. Being L$0 means that you won't have to pay to try the styles to see if it works on your av. Once you've found a demo that you're happy with, check your colour charts and then you can make a purchase with confidence knowing that the product will work for you.
Enjoy shopping!