The reason they use other people's money is for leverage. You lend them money and they give you back the principal and a small percentage above that. What they get out of it is that they make a lot more on top of that.

This is the basis of any banking system. When you place your money in a savings account, the money is sitting in a vault, the bank is investing it. While they'll return 0.25 - 1.0% annually to you, they make 4 - 10% on that money for themselves. Multiply this by millions of customers and this is how banks become so big.

BitConnect is utilizing this same law but for the Blockchain.