Why lead object is used in Salesforce?

Lead is nothing but showing interest to buy the products/services/goods, but not customer until convert to customer from lead. Here, if we have not maintain the leads then possibilities are there all the potential customer details also stored in Account. How to come to know who is the actual customer. For that, after paying initial amount, then only we can consider as Account. Hence to avoid to store all the potential customer details along with actual customer details in account we can use to Leads. After paying some amount for particular contract period then only we can say our actual customer. Then after we can convert to account from lead.