IMO, It is mostly just a mix of document repository (SVN) and custom website with permissions linked to active directory. However, organizations that can't roll their own intranet site worth a damm can often setup a SharePoint site that meets most of their needs. It supports customization through both plugins and custom code which is how MS get's their long term lock-in. For companies that can maintain say a talented Ruby on Rail's team there is little benefit but where the 'programming team' mostly slings excel macro's SharePoint can still get stuff done.
It's a typical buy vs build scenario. From my understanding, it's not difficult to integrate other enterprise apps to Sharepoint and make a dashboard.
You can probably do that with a bunch of Rails people but if the platform provides a decent "Portal" capabilities out of the box with minimum modification, why build from scratch?
The alternative is to install MediaWiki, WordPress, some sort of CMS perhaps, some sort of Portal (LifeRay?), some sort of document management system that supports versioning and decent web UI, integrate a bunch of services using custom web-app. That's a lot of infrastructure to deal with.
What do we need Excel for when we have perfectly good Perl and CSV files, you might as well ask...
Whether that is worth what Microsoft is selling Sharepoint for, well, that depends on the buyer. It also has the "nobody was fired for buying IBM" point (and Microsoft does have decent support).
I still haven't seen an Open Source Sharepoint "killer", though I might not know of it (I had good hopes for Alfresco).
So it's both FTP + SVN rolled into 1 feature set under SP umbrella.
Sharepoint appears to be little more than a recreation of the Internet "in the small". First time I saw it I had the same thought as the last time I saw it: "WTF do we need _this_ for? We already share files; we already have FTP; we already have the Internet. WTF???"
Am I wrong about Sharepoint?