We used fast.com to speed test our new office internet connection and the next day got an irate email from corporate (who had argued we didn't need the new connection) about "watching Netflix all day". I imagine some C-level thought they had a real gotcha! moment until I showed them the site.
This is another advantage of fast.com.
I read a while ago that certain ISPs will optimize the traffic to Netflix's servers, and so when you run fast.com (which is my default, by the way), you get your Internet speed for watching Netflix, but not necessarily for other things.
Out of curiosity, I just compared my home wifi between fast.com and cloudflare's speed tedt and got similar results, completely and definitely disproving (n=1) my claim above.
That was very relevant in some scenarios. When Spectrum was fighting with Netflix, they would force Netflix traffic to a peer circuit that was under provisioned as a shakedown tactic.
Fast.com would detect that, and you could bypass that nonsense by changing your DNS.
The opposite can also be true. T-Mobile throttles Netflix and fast.com on my 5g mobile plan to be less than 5Mbps where Speedtest shows > 200Mbps.