A PhD thesis needs to have a significant, original contribution in it. So a postgraduate researcher needs to make a significant, original contribution for it. Significant, original contributions (to research, to knowledge, to the world) don’t grow on trees. They don’t fall out of the sky. They don’t happen by accident.
If you’re feeling nervous or even afraid at the end of the PhD, weigh it against what you’ve done to get this far. The dedication and work you’ve put in tip the scales in your favour.