What is the purpose of building a client library? [on hold]

I'm currently developing a client library for Gengo in Go. There are many resources on how to build a good API but I haven't found anything on how to build a good client library.

Some developers I've talked to argue that calling an API endpoint is essentially just a cURL, so all the responses should be json strings in the language the client library is written in. That way a developer using the client library can then cast the json response into any type they want.

Isn't this the original purpose of building client libraries, to make it as easy as possible to use an api? In this case, doing the casting for them? I'd like to follow some standards.