As each individual deal moves through each phase, the probability of closing the sale will change. The further along the sales funnel, the more information is exchanged and more apparent it becomes using the product will be advantageous to the customer. At this point, there is a higher potential for that deal to be ultimately successful unless the deal is moved to closed-lost in which case, the probability moves to zero.
In marketing automation, Ryan Deiss, co-founder of Digital Marketer, often describes the sales funnel as a multi-step, multi-modality process that moves prospective browsers into buyers. It's multi-stepped because lots must occur between the time that a prospect is aware enough to enter your funnel, to the time when they take action and successfully complete a purchase.