This often gets listed as one bullet point for Android, and indeed it's a great one, but I don't think other people realize how huge this one point is.
It can literally change the way one's phone is used. Say for example, setting a different default keyboard. Going forward, this changes and affects everything that involves typing (which is almost everything): entering new contact information, writing emails, inputting web addresses, sending texts, writing notes, etc. Changing the default browser means every time you go ont he web, it's now that browser's specific strengths and features.
Etc. etc.
It's a huge point. Everyone loves to point out the App Store's exclusive games and apps, or its number of apps over the Play store, but it almost begs the question, what's the point if it can't be set to default? Likewise, that means the Play Store has an insane amount of exclusives too that iOS could never have (like certain keyboards, launchers, etc.)