I spent about ten minutes on the site, so hardly a domain expert. I’m still confused, too. But as best I can understand, you have the option of storage on S3, Azure, and the like. I assume that with a plug-in/driver, you could store anywhere you like.
But non-local storage does seem to be designed in, because there is text like “if there’s a daemon running rsync in the background, you’re doing it wrong” and “if your UI requires marking folders to be synched/not synched, it’s broken”, so there appears to be an assumption of putting your data elsewhere.
It's like your own personal google drive / dropbox / git repo
It's a content addressable storage system. There's plugins to import or export from various major services like foursquare, twitter, etc. and plugins let you store stuff in S3 or mongo or google cloud storage, etc.
I suppose technically running something like OwnCloud with a plugin to fetch your content (text/photos etc) from the various social networking APIs would look near identical from the outside?
Yeah, that part is confusing. If you can throw local drives at it plus a cloud service or two, and it'll just make backups on the cloud, that's actually pretty interesting. It's not clear if that's what's going on though.
As much as possible the design attempts to be agnostic about where the stuff is actually stored. Usually you want to be storing your stuff in multiple different places. Among others, Perkeep lets you choose one or more (or all) of the following:
Your own hard drive local to the Perkeep instance
Your own hard drive local to another Perkeep instance you run
Your friend's hard drive local to another Perkeep instance _they_ run with encryption to ensure privacy
A removable hard drive you can periodically sync to and from
A cloud service like S3, etc
It has sync and backup options, tools for non destructive changes and robust search, a web UI so it can be used like a dropbox. Just because it's not useful to you doesn't mean it's useful to no one. Oh and importers for cloud things, they show off automatic sync pulling down a twitter feed and some foursquares check-ins.
Not an effort to sway your opinion, just pointing out that it's not really as simple as just saving images to a hard disk.
I am so confused by what these people do.