Why are asynchronous runtimes like Tokio necessary?

Question

My first experience doing a computer system project was building a server using vanilla Java and then a client on an Android phone. Since then, I've found that there are a lot of frameworks to help manage scalability and remove the need to write boilerplate code.

I'm trying to understand what services like Tokio and Rayon enable.

I came across this paragraph on the Tokio tutorial page and I'm having a hard time understanding it

When you write your application in an asynchronous manner, you enable it to scale much better by reducing the cost of doing many things at the same time. However, asynchronous Rust code does not run on its own, so you must choose a runtime to execute it.

I first thought a "runtime" might refer to where the binary can run, but it looks like Tokio just provides functions that are already available in the Rust standard library while Rayon implements functions that aren't in the standard library.

Are the standard implementations for asynchronous functions written poorly in the standard library or am I not understanding what service Tokio is providing?