"By default" means the app owner does not need to give permission or do any work for an iOS or iPadOS app to appear in the Vision Pro's App Store.

How much of an advantage does this give the Vision Pro over competing products / ecosystems?

I think it’s a pretty major advantage.

For developers, it means that you’re not writing a completely new app for the platform. You can progressively add features for the new platform to your existing app, and most of what you learn applies to other Apple platforms.

For customers, it means you aren’t taking off the device to use an app that’s not been ported.

apple is trying to extend the ecosystem, so anything to make it more sticky is a positive.

eliminating reasons to use a different device increases stickiness.

this guarantees people at least won’t have to (a stretch to assume they won’t) touch an iphone or an ipad while the headset is on their head - the same apps are available, and any using icloud for storage may simply be “the same.” same data, same app, different screen.

advantageous, yes. advantage over others? less clear.

Meta could put Android apps on the Quest.

It just does not want to, but maybe this will change their mind.