While I agree with everything you said, I still think providing a downloadable version in addition to the one that runs in the browser has a purpose: preservation. If itch.io were to die someday, all the browser-only games will die with it. If people can download them locally, they can survive virtually forever. Not to mention that it also allows you to play them when you don’t have Internet access.
I, for one, love preserving all forms of art, games included. I still have flash games from the 2000s kept safe in multiple hard drives. While it is technically possible to archive HTML5 games, people will rarely do it as the process can get quite involved.
But yeah, in the end it’s your choice, just giving you another PoV :)