Liferea is not the only aggregator with folder-like organization, but it is true many aggregators rely on flat category-based organization. And there are some not even supporting categories.
To be honest in the beginning when adding folder support I just followed the email client interface, not thinking about better alternatives. But after some time working with folders I found it to have some advantages:
Like seen in the above screenshot when the user is allowed to select a folder the aggregator can present a merged item list. Imagine all 10 feeds of the folder posting 1 post per day. With a category based approach you are forced to click each feed once - as long as there is no merged mode presented, which would have to be exposed by some menu or toolbar option.
The same when marking every of those 15 post feeds as read. In Liferea select the folder and mark the folder as read. No extra menu option for marking the whole category as read.
Also there is no need to expand the folder. If you know about the contained feeds anyway and can identify them by their favicon you can have a pretty small view of a handful of collapsed folders (maybe labeled by category type) which you click one after another to read their new content.
Of course there are also the disadvantages some of which the Blam! developers already identified:
- One has to explicitely collapse folders or scroll to access of the rest of the feed list. Maybe Liferea needs an auto-collapse preference.
- Folders realized with GTK tree views eat up vertical space. Each indention level eats about 16 pixels.
- When using folders as categories one cannot have one feed in two categories.