POSTing with curl's -d option makes it include a default header that looks like Content-Type: application/x-www-form-urlencoded. That is what your typical browser uses for a plain POST.

Many receivers of POST data do not care about or check the Content-Type header.

If that header is not good enough for you, you should, of course, replace that and instead provide the correct one. Such as if you POST JSON to a server and want to more accurately tell the server about what the content is:

curl -d '{json}' -H 'Content-Type: application/json'