Why does the async keyword exist

I think perhaps this article covers the reasoning:

https://docs.microsoft.com/en-us/archive/blogs/ericlippert/asynchrony-in-c-5-part-six-whither-async

The first paragraph states:

A number of people have asked me what motivates the design decision to require any method that contains an "await" expression to be prefixed with the contextual keyword "async".

It concludes:

That's a whole lot of pros and cons; after evaluating all of them, and lots of playing around with the prototype compiler to see how it felt, the C# designers settled on requiring "async" on a method that contains an "await". I think that's a reasonable choice.

The short of it is backwards compatibility.

Further reading:

http://blogs.msdn.com/b/ericlippert/archive/2010/10/29/asynchronous-programming-in-c-5-0-part-two-whence-await.aspx

It was introduced mainly to avoid backward compatibility issues. If the async-ness of a method must be inferred by the compiler (that would be through the detection of await keywords), then there are subtle scenarios where existing code would suddenly be treated differently, notably when you have identifiers (variable or function names called await).

A full explanation is here: https://docs.microsoft.com/en-us/archive/blogs/ericlippert/asynchrony-in-c-5-part-six-whither-async