What's the difference between ob_flush() and flush() and why must I call both?

The ob_flush() reference says:

This function will send the contents of the output buffer (if any).

The flush() reference says:

Flushes the write buffers of PHP and whatever backend PHP is using (CGI, a web server, etc).

However, it continues to say:

[it] may not be able to override the buffering scheme of your web server…

So, seems to me that I could just use ob_flush() all of the time. However, I get strange results when I do that. Could someone explain in simple terms what's going on here?

php flush output-buffering
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