Ever since mankind was able to make more arrowheads or hides than their family could use, mankind has been looking for someone to barter with. Where there is trade, buyers as well as sellers need referrals and leads. No sales person has to be told that if they don't have referrals, they spend most of what should be their prime selling time looking for someone to make a presentation to. When they do find someone who will listen, they still only have a 'cold' prospect, and they usually run out of sales time before they make enough presentations to make a living. Prospecting is the most difficult part of the sales process, and is also the main reason that people fail in sales.
Time management is crucial.
You usually have only about 7 hours each day to make sales calls. If you have to spend that time prospecting, you are wasting valuable time you should be spending in making presentations. By the time they find someone to make a presentation to, they're tired and discouraged. If you have been in business for several years, and have established yourself as someone who gives and gets a steady stream of referrals, you may not need any more of a referral system. Selling to consumers is a little different.