- To determine if an item is new or not, allowing the authors of weblogs to make minor editing changes without making all their readers figure out if a post is new or not.
- If it's a permalink, make it easy for the reader to go directly to the item on the web. This is cool for people who want to quickly include the link in their weblog.
The second feature is a nice convenience, the first, imho should be a feature of all aggregators, readers and content systems.
My aggregator, when it reads Jenny's feed, displays a little purple pound-sign -- the standard graphic for a permalink -- next to Jenny's posts. This way I can jump directly to the archive page that contains the item I'm reading with a single mouse click.
Here's a screen shot that shows what Jenny's post looks like in my aggregator.
Also, because her weblog tool supports this feature -- and my aggregator does too, if Jenny edits a post, corrects the spelling of a couple of words, or changes the phrasing in a sentence, I don't have to read it again. I'm really busy, and so are many other aggregator users, and appreciate the time savings.
Guids sure have a funny name, but they're quite useful. If your weblog tool supports them, use them. If not, ask the developer to add the support. It's not very hard. Further, I strongly believe that all aggregators and readers should pay attention to guids where they are available. It's a convenience that many users will appreciate, especially people who are in a hurry.