why do RA developers sell pre-construction units at a discount?

[Patrick K.]

Hi all, I am curious on why do RA developers want to sell pre-construction units at a discount? 

I understand in some regions, developers must pre-sell a certain percentage of their units before they can start the construction. What about in areas that do not have such requirements. My laymen's guess is that they need the deposit from the pre-sale for the equity portion (usually 25%) of their construction loan? Is that the case? If so theoretically, other than the initial building permit and design soft cost, a developer can utilize the end purchasers' cash for equity and not put in a dime of their own money? 

Thank you for your time.