One thing that's nice about a monorepo is if the language has modules. It it supports modules well you can isolate your code better but still keep it in one large repo.

I dislike having all my code under one 'src' directory and it's nice to have modules like foo-ui and foo-util and so forth. Knowing that one module doesn't use ui components is nice because you can use it on the backend for example.

But it's all fully integrated and tested together.