It doesn't. It's a standalone (static) helper method that uses HttpClient to perform the request and feeds the response body into a json parser. It's an extension method [0] which means that there's syntactic sugar so that you can write client.GetFromJsonAsync() and the compiler transforms it into the actual static method call, HttpClientJsonExtensions.GetFromJsonAsync(client).
Mostly because JSON is one of the most common formats used when sending over data. I think this practice started with pythons requests (which is my real answer as to what you should use in python but I wanted to focus on the stdlib), which has a json function on the Response object for convenience.
Most languages nowadays tend to implement some variation of this specific convenience because it's just one of the most frequently needed things; setting up a separate parser and then calling it might be the "cleaner" option, but it's also more boilerplate and the industry has largely moved to try and avoid that.
