This is what I was going to ask as well. The only benefit I can see to this is that they are trying future proof for when WebAssembly or some other back end is more prevalent? Maybe they want to do Glimmer-Native?

Handlebars -> JS/TypeScript compiler -> Bytecode -> JS probably anyway -> HTML seems like a lot of work.

The bytecode representation is more compact than the compiled JavaScript. And this has an impact on execution speed.

See past discussion: https://news.ycombinator.com/item?id=14050625

I would assume because their VM/bytecode version is very fast at rendering updates and smaller in size than a "represent the bytecode as JS" version.