From a brand perspective there are some pros to walled gardens which may also benefit your users:

1. You get to have a fairly uniform app experience. Stricter design guidelines allow the company to have more control over the UX. You grow to expect a certain level of experience from App Store apps. On Android it’s a hit or miss. Some of that on apple’s OSes too, but not as much.

2. The App Store can have more quality options due to #1, and you don’t have to have a place full of garbage (as in a lot of cases with Android).

3. Brand exclusivity / positioning. Say what you will, but there’s value in feeling like you’re part of an exclusive club and no other hardware features your software. (Thinking about iOS in particular because you can make OSX work on non-apple hardware).

4. With brand exclusivity you can charge more, in effect reinforcing your brand and positioning. This helps you build a more loyal customer base.

5. Ability to be more thoughtful in what they ship. Instead of chasing the next new thing and shipping as fast as possible, 1-4 allow you to refine the existing new thing and make it a better experience.

Apple has proved for a while now that there is a market segment of people who value these things above better hardware or more features. In a startup’s sense, it’s equivalent to doing one thing right vs. many things so-so.

Obviously there are downsides as we all know, but these are all calculated decisions.