you use them when you create a lot of accounts on social media and other website and not get banned for sure (there is still the possibility) . You need them to show those website it's not just you creating a lot of accounts, but they are coming from different IPs that i think use the proxy. I'm not an expert and i need clarification too but this is what i know. You also don't mae money with them, the only way for that is to sell them i suppose...but you can use them to create more account and so more followers,friends ext. and so more probability to earn money.