This is a very simple feature that allows you to be able to assign a value to all or just some of your deals at any stage of your sales pipeline, and thus be able to calculate the combined worth of that particular stage of the funnel.
This information is private to you, and your customers are not aware that you have assigned a value to their deal.
Also, the deal value number is different and not related to the proposal value you may send out. This is set up like so because you may be quoting for a single project, but the whole deal is worth much more because of the type of project (e.g. the deal is open-ended, but you are sending a proposal/quote for the first month of the work.)