Let's start out with a definition. For those of you who aren't sure, this is what a "feed" is (when it comes to websites):
A web feed is a data format used for providing users with frequently updated content. (Wikipedia)
Let's take a closer look at this concept. The key idea here is "frequently updated content". A website that features a blog or updated news or any type of content that changes regularly will want to offer users a feed. This is because most users will not want (or remember) to visit every such website every day, and users will lose track of which websites have new content today, which don't, what they've already seen, and so on.
Instead, they'll use a feed aggregator (or feed reader). The users tells ...