But i don't really understand the usage of the bitcoin.
Is it just to stay anonymous buy using it to pay or is it interesting to earn money buy buying and selling them ?
Most often people use bitcoins to pay anonymous and for services / products they can't buy for instant with PayPal. Investment, maybe, I don't know how this works.
There are many people around that have largs ums in their bitcoin wallets as far as I know and that are into day trading bitcoins. If you are good (more lucky) you can do very well with larger sums.