Leads are created from contact records (Create a Lead Record from a Contact) and are a way to track potential business before you have a scope of work (to start a project) and before you want to start involving other users in your company in the process. Anyone can create a lead record as long as you have access to the contact record.
Lead records also track your marketing efforts using Marketing Campaigns.
Even if you are just going to create a project directly, you should create a lead if the customer (or potential customer) came to you by way of a marketing effort. This will allow your managers to evaluate whether a marketing campaign was effective or not and whether it is worth doing again.
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