Using vague terms like "the platform" are not useful. What does an operating system like "iOS/iPad OS" have to do with the "iOS/iPad ecosystem"? You're not clear even in your own reply. Please try to be more precise.
I'll assume what you're really asking is from the perspective of Apple since the benefit for users is clear - it'll help make more apps available which has network effects of more Apple devices and usage.
This is part of why they are being forced to allow sideloading in the EU. As it turns out in the real world, the interests of corporations are often at odds with the interests of every day people.