The biggest usage is probably not the tool itself, but the library. It's the de-facto-standard-HTTP-library (although it does more than just HTTP). E.g. if you

The biggest usage is probably not the tool itself, but the library. It's the de-facto-standard-HTTP-library (although it does more than just HTTP). E.g. if you have a PHP script that downloads some data from another webpage it very often does this with curl.