Free (or discounted) copies are provided to provide "buzz" - ie, for people to experience it, and then share their enjoyment.
HOWEVER, I work in marketing, and there's a very important point from his field about "perceived value".
Simply put, anything "free" can be assumed to have no value unless there is existing perceived value.
If Neil Gaiman offered a new book for free, then going by his track record, many people would presume his free book would have value, and therefore rush in to grab it.
If an unknown author offers a book for free, then there is no existing perceived value, so not only may people not care to download it, they may also ensure it's pushed right down their reading list.
"Free chapters" get around this somewhat - they are, by their nature, samples of something that claims to have value. It is up to whoever "samples" this as to whether the product really does appear to have any potential value or not. This is precisely why Amazon do their "look inside" feature.
Hope that helps!