Why turn header off in curl?

As per the docs, it controls whether the response header(s) will be returned alongside the response body. Generally, if you only care about the response body, you want this disabled (which is the default value, 0).

When CURLOPT_HEADER is set to 0 the only effect is that header info from the response is excluded from the output. So if you don't need it that's a few less KBs that curl will return to you.