For this answer I'll assume that you already know what a WebID (formerly FOAF+SSL) is, and all the benefits which are entailed by giving each person a unique personal URI, which deferences to machine readable (linked) data using the standards, the benefits of a linked data based decentralized social web and all related. These all have end user benefits which I know you already know hence not included.
More End User benefits to you: FOAF+SSL natively requires that all communications whilst identified occur over HTTP+TLS, this means that all personally identifiable information is always sent encrypted across the wire (protecting you against sniffing, deep packet inspection, man in the middle attacks, and related). It ensures that all connections whilst identified are between two identified parties, and this is further restricted (by HTTP+TLS) to being a direct connection between the two parties that can't be tunnelled or passed on over more than one hop. Further still it ensures that the vital identifying 'secret' usually a password, but in this case a private key, is never ever transferred over the wire and doesn't leave your user agent.
Technical benefits to the web as a whole (and thus to end users) From a web architecture point of view, FOAF+SSL is a stateless authentication mechanism; one of the biggest problems on the web is that HTTP is a stateless protocol which 99.9% of people use as a stateful protocol, using sessions, keeping you 'logged in' and such like; this brings huge performance, caching and security issues to the web - FOAF+SSL negates all of these by being a stateless RESTful authentication and identification protocol, so inbuilt it's good for the web, for performance (security as mentioned) and moreover, it encourages good practise when developing web applications and makes the web entirely more RESTful.
These are points you can play down, but they are the root of the majority of the problems on the web today, big fat important issues which impact almost everybody on the web in countless ways - these alone are more than reason to adopt FOAF+SSL.
Daily End User benefits As mentioned there are all the Linked Data, Semantic Web and Decentralized Social Web benefits to boot (which are huge themselves) - then there are the simple things which make a big difference on a day to day basis - no 'logging in', you are either authorised or not, no having to remember passwords, no complicated sign up procedures, all profile information in a single place so no having to refill in all the same details over and over (and update them when something changes), no dependency on all consuming corporations that monitor your every (social) move (which they do via controlling your logins so you are always routed through them via openid, 'share this', 'like' etc), no NASCAR problem (list goes on)..
Hope that helps a little
re: 'I get many, if not all, of these benefits from having an OpenID, e.g. data about me can be accessed via profile exchange. OpenID can also be done over SSL. So what is it that FOAF+SSL/WebID actually adds?'
- OpenID doesn't require the use of TLS, FOAF+SSL does
- OpenID requires transferring account authentication details across the wire (and 'logging in')
- OpenID natively encourages a stateful anti-pattern in the web which gives performance, caching and security risks.
- OpenID is open to phishing.
- OpenID is driven by the big players, and it's their tool to 'lock you in' to their networks, see janrain as the perfect example (also showing the NASCAR problem).
- Public facing profiles are anemic, and although there is a semi supported attribute exchange it is extremely limited when contrasted with full RDF/Linked Data and RDFa profiles with unlimited scope.
- ... many many more points.
What does FOAF+SSL give?
Simply, FOAF+SSL wraps up all the good things on the web in to one package and requires the use of them all, it requires the use of HTTP+TLS when identified, it requires using Public/Private keys via an X509 certificate, it requires the use of Linked Data, it requires the use of Standards, it requires that each person has a Personal URI / WebID, it requires that personal URI to be dereferencable, it requires cutting out the third parties and middle men. In built is a decentralized and federated secure social network which can easily be supported on all devices that support https. It puts the clientside part of authentication in to the APP which controls HTTP, in other words the browser, thus removing the need for convoluted 'login' systems to be included in applications.
FOAF+SSL allows (and is imho the key enabler in) the read/write web of linked data, it enables a paradigm shift where applications run on the client and data is stored on the web; it brings another level of universality to the web, it..
I'd ask, what's the reason not to use FOAF+SSL?
Edit 2 RE: 'What's the end user benefit over OpenID?'
- All information you send or receive whilst identified is always encrypted
- You don't need to 'login' or remember passwords and usernames any more
- Your profile is ultimately extensible, machine readable and unlimited.
- You are't faced with multi-step login processes which are plastered in logo's
- You use an authentication protocol which enables a new generation of web applications gained via a web revolution and huge paradigm shift. (which other protocols don't support because they require server side web applications and stateful authentication)
- Can be used for more than just humans, any Agent or Application or Device can have a WebID in this manner
- Not limited to the Browser/User Agent, WebIDs are generic to the web and HTTP(+TLS)
all the other benefits already come from having a personal URI, a FOAF profile, being part of the web of linked data and similar :)