I worked at Linden Lab (Second Life) for 5 years, and still think that SL is one of the most fascinating things on the internet. But very little of what makes it fascinating is easily communicable as a simple explanation of "what the metaverse is for". (Also, hi! Not sure if you remember me but we hung out occasionally some years back. Noisebridge, Burning Man, etc.)

Firstly, let's drop the term "metaverse" in favour of a basic definition: user-created, shared, persistent 3D worlds. Kind of like our world, the real one. What's our actual real world for?

It sounds like a ridiculous question, so let's get something out of the way: I'm not pretending that Second Life and its ilk have even 0.001% of the richness of real life. What makes them worthy of this question is that they're closer to the flexibility and expression of real life than other shared experiences we can have with the same technology.

Keep that question in mind and I'll talk about something more concrete.

For use cases, let's kick enterprise "work" usage out of the conversation. The vast majority of 3D world applications are either pure games or game-like enough to still be thought of as games, even when a big chunk of the usage is more social. (For example, all the people on shared Minecraft servers just hanging out, being silly, spending time with friends, building together, and not trying to win at anything.)

In my experience, we don't give that social usage nearly enough credit, but it's much harder to see and measure because it has no consistent shape. You could call it "people using a flexible toolset to enjoy new ways of interacting". If that phrasing sounds far more serious and deliberate than what most of us see, how about: "people messing around and having fun." Either way, it's valuable enough that hundreds of thousands of people do it and love it.

So, user-created 3D worlds are a way of dropping all the game-imposed mechanics and constraints, and giving the users far more control over the environments and tools they can use together to mess around and have fun. Or create amazing and terrible art. Or share fashion and furniture and architecture and things that make it sound like The Sims until you remember that fashion is a massive industry in itself and being able to participate in it, even just "virtually", is still intensely desirable for a large number of people.

3D worlds are Platforms, not Applications. There are tons of applications already but if you want a neatly encapsulated Killer App, I don't know that you'll find one. At least, I don't know that you'll find one that suits you. This is a big part of what makes this so difficult.

The ideal of a killer app is one that you see a five minute demo and you're sold on the entire platform. "Ah, now I can do this thing that was nearly impossible before!" VisiCalc was that killer app for personal computers, but a key component of its success was having a large, clearly-defined target market: financial planners.

The target market for 3D worlds is far harder to define. It's about social desires and needs.
However, in my years at LL, there was a persistent quality I found that became more and more noticeable over time, and which most people trying to sell "the metaverse" don't talk about because it's taboo. And no, it's not sex.

There is a huge chunk of the Second Life userbase who, for reasons mostly out of their control, have little social life outside of SL. Maybe it's due to location, or physical disability, or social anxiety, or a bunch of other factors. (It's usually a combination.) I don't know if this applies to the majority of SL residents, but it's close.

Whatever the reason, SL gives them more control over how they present to others, both in appearance and in that magical extra 10-20 seconds you get for typing instead of talking. This increase in control is enough that they are able to have active social lives with people who they care about, and who want to spend time with them.

That's a killer app right there. It's not the killer app for everyone, not by a long way. Everyone gets something different out of it, and there are huge numbers of people for whom 3D worlds just aren't ever going to appeal. (Just like spreadsheets.) But there's far more actively-enjoyed value here already than you might otherwise see. You just have to find the right platform, activities and people to enjoy it with.