Because it's useful in aiding in development. "Telemetry" is not "spying" or "collecting personal information" or whatever else uninformed people like this blog author try to dramatize it as. Telemetry is anonymous crash reports, how many users are using vertical vs horizontal tabs, how many people engaged the AI killswitch, etc. If it's NOT anonymized/bucketized usage date, then it's NOT telemetry. People need to stop conflating the two.
Ironically, it's the people who disable all telemetry out of ignorance and paranoia that end up crying every time a developer deprecates their favorite feature or they have an issue with a new release. If you don't vote you don't get to complain. Mozilla's PM's in particular have repeatedly published and cited telemetry data in their decision making, and have referenced crash reports in proactively filed bugs.