I didn't see any difference with or without this head information yet.
It is necessary if you want the browser to know what the type of the file is. PHP automatically sets the
Content-Type header to
text/html if you don't override it so your browser is treating it as an HTML file that doesn't contain any HTML. If your output contained any HTML you'd see very different outcomes. If you were to send:
Content-Type: text/html would output:
Content-Type: text/plain would output:
TLDR Version: If you really are only outputing text then it doesn't really matter, but it IS wrong.