NitroPack is a full stack page optimization service and not a hosting solution. It works on the top of the already existing (often marketed as "fast") web hosting providers.
Having a good and fast hosting is great, however this rarely means fast site for your visitors. A typical super fast hosting would typically be running generic publicly available lower-level cache tools, however it won't optimize the application and presentation layers code of your website. This is where NitroPack kicks in and boosts site delivery and browser rendering. Therefore when testing your site with bench-marking tools like Google PageSpeed Insights or GTmetrix you get much better scores when running NitroPack.