Offering free days has become a fairly common way for new authors to promote their work. Kindle has several classic books no longer up for copyright that you can download for free. And you don't necessarily need a Kindle to take advantage of these free books, since the Kindle shop also has a free app for PCs and other devices.
New authors do this to let the public sample their work, so if you like a new book that you download for free, it's nice to let other people know that you liked the book. Not required, but most authors (even those with traditional publishers) rely heavily on word of mouth for promotion.
I don't think ebooks are going to entirely replace print, but I think we're going to see an ebooks first trend increase. This gives publishers an environmentally friendly and cost effective way to see if books get a strong enough public reaction to support a print run.