Excellent question. Thank you for asking.

Maybe a similar way to think about it would be task "What does the STEEM blockchain actually do?" OR "What does the BitShares blockchain actually do?"

By themselves, they do nothing. They are just decentralized, globally distributed, immutable, cryptographically-secure ledgers (i.e. databases). I know, that's a mouthful for "street level," but those are the best words I've found so far to describe what's going on.

Now, when you look at the Bitshares wallet/decentralized exchange or at Steemit/DTube/UTopianIO/DLive/etc... then you might start to get a feel for what's possible. The EOS blockchain itself is more like a protocol. TCP/IP is just a protocol, but it enables the entire richness of Internet we all enjoy. SMTP is just a protocol, but it's what makes email work. EOS can be looked at like the Amazon Web Services (AWS) of blockchain technology. Much of the internet runs on AWS right now because they have a huge economy of scale with proven tools that have worked for a very long time. EOS, in a similar way, will enable corporations around the world to have enterprise-level access to blockchain technology to build the systems of the future. It can deliver on promises other blockchains have made, but can't handle (such as Ethereum crashing due to cryptokitties).

Dan Larimer is building EOS because he wants to build the low-level protocol that he can then build everything else on top of. Every aspect of society and governance that will be improved via distributed ledger technology could then be built on top of EOS. It's a really exciting time to be alive. :)