Use leads if you need a qualification step before creating an opportunity or a customer. It can be a business card you received, a contact form filled in your website, or a file of unqualified prospects you import, etc.
Once qualified, the lead can be converted into a business opportunity and/or a new customer in your address book.
Typically, this is the beginning of the sales and crm process. You will make use of the system to keep track of all the business contacts you have and look for opportunity to proceed further.
However, you can also skipped this step of creating a lead if you can qualify the customer during your first meeting. You can create a opportunity directly.