The biggest advantage is that it already does a bunch of the work for you, which means you have to do less wheel reinventing.
For instance the file storage package, CMS and photo album are all built around a hierarchical folder/file paradigm. This come more or less for free if you use the content repository (which includes file and folder content objects).
The CR also implements revision tracking for client packages, and in the Oracle case hooks into content processing stuff provided by the RDMBS.
Now ... the toolkit as inherited doesn't leverage the CR as much as it should. For instance each of the above mentioned packages allows for file uploading and display, but the CR gave no help there. That's something I've been working on but have yet to incorporate throughout various packages. Where I have, though, big chunks of code have shrunk to little itty-bitty calls to my additions to the CR's Tcl API.
Another advantage of having it all in one place is that central services like searching, categorization, etc can all work on a common datamodel. They don't have to "know" if package A implements revision tracking while package B doesn't, etc etc.
It's all about writing less code when you invent a new package, and not having to deal with so many special cases when you invent a central service.
At the moment in some respects, at least, it's more in theory than in practice, but with practice it will hopefully come to equal theory :)
