No mystery to RSS, it's simply software that allows you to keep track of when blogs, news sites, forums etc have new posts. Advantages are you are not tied to a particular device (as with browser bookmarks), nor a particular blogging platform (eg. Blogger). I use a RSS reader to keep track of both personal amusement (web fiction sites) and professional blogs. I'm surprised more people don't use them. Used to use Google Reader until Google decided to discontinue it, now I use Inoreader, a freeware I chose for its simple, clean interface out of the numerous options out there.
To address Lee's original question, I believe the majority of sites do provide the full feed i.e. you can read the entire post in your reader. An example is Earthcast whose feed is http://www.koryoswrites.com/feed/. Here's an example of a Wordpress blog that provides the full feed: http://betterknowachildballad.wordpress.com/feed/
However others do provide only a "teaser snippet" the first few sentences. I suppose the advantage of this for the blogger is it forces you to visit their site so your page views are recorded. I don't know how this is determined, whether you get the full feed or the snippet, but it's something on the host site's side.