I have used Eclipse since version 2.x and am VERY familiar with it (had at one time worked on an RCP application in a job and even wrote an OSS plugin more recently). I use it mostly for core Java development and C/C++ natives that go with that Java. Recently I'm using for Python as well with CodeMix.
I'm in my 40s and been coding since 13. I've also used IDEA and briefly PyCharm (switched to VS Code and use that for Python) and of course Visual Studio when I was working for a Windows C++ application. I spent quite a few years with vi (nvim later) for C development. So I've been around.
My main drivers currently are Eclipse for Java and VS Code for Python. For C/C++ I use both depending on what I'm developing natives for. I also use Atom quite often.
I've always considered Eclipse superior to all other stuff I've used. But for the average Joe, it is daunting.
Here's my main answer to your question "what do you use it for, and now that this is announced why would you pick it over VS Code?". Personally I like VS Code and I've long considered it better for Python development than Eclipse. Just as I've always considered Eclipse better for Java development than VS Code. However, even before Theia, I started recently evaluating CodeMix so that I can switch fully to Eclipse.
The real thing is: both can do more or less everything you need, but Eclipse just does quite a few things better in Java.
My feeling is that the real problem with Eclipse is ACCESSIBILITY. If you go to eclipse web site there's so many alternatives (for Java, for JavaEE, for C/C++, for PHP), so many nice features must be added via the market place (or update sites), it lacks a nice dark theme (which is all the rage past few years) and so on and so on... Go search for markdown or YAML support in Eclipse marketplace and you won't know what to pick...
The way I see it, is: Eclipse has fallen victim to IDEA, VS Code, etc. because: they look nice and work nice out-of-the box. In comparison, to make Eclipse usable I have a *python script* that installs cherry-picked extensions by myself from various sources. If you know what to install, it's the best way to quickly install Eclipse. But a new user will just go to IntelliJ for Java and VS Code for Python.
Eclipse IDE is like Linux distributions, only worse: with Linux at least there's Ubuntu (and Fedora) that are a bit of 'de facto' standards, but in Eclipse there is no single source offering a curated distribution that works well and also LOOKS well. Genuitec seems to have done something nice with DevStyle which I use for good looks. They've also released CodeMix recently which I'm currently evaluating for Python and so far I like what I see. I might ditch VS Code and go to a pure-Eclipse installation (if I like it after using for a while).
Every year Eclipse gets a bit more 'last year' and eventually it will die, which will be sad...
Hopefully it will keep a core of knowledgeable users to become the Emacs of 2030-2040.