My mormans friends,why?

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I was in switerland before, I found many mormans who work there. They are in black and they are kind, most of them from america, some from German and other countries. At the begining , I was just cuirious about what they want to do, for me ,a guy from a mainland where there are not much people to have a religion, I just try to approach them, during our time together, I learned a lot from them,but what I understand happened at last,they told me God give us everthing, as a return, we should give 10% of we have for the cause of god. A question I thinked a long time, why ?