How does CAC for e-comm industry compare via affiliates vs other means?

I would like to know the perspective in different countries and different niche.<p>I have been hearing that the customer acquisition cost(CAC) is high for most e-commerce companies. Does affiliate marketing help in reducing the cost? How so? If not, then why do websites start affiliates or partner with affiliate networks?<p>If an existing customer comes via an affiliate, how does this impact CAC?