All app stores, IMHO, are about app discovery for people who want to search as little as possible. The iPhone app store is the only way to get apps "legally" on the iPhone, but other than that, app stores don't do a whole lot other than search. Android apps can be sideloaded, Mac store apps won't be anything new, and Chrome apps aren't either.
Having a button for the average user to press is what it's all about. "Press this button, and you can find apps." It just serves as a way for people to most quickly find something that meets their needs. For developers, it gives those people a place to look and hopefully increases exposure.
App Store for smartphones is more about lock-in than app discovery. If a consumer has too much invested in iOS apps and games for example, his/her next phone upgrade will most likely be still iPhone.
Apps cost so little compared to a phone + 2 year contract that I doubt this is the reason. You are certainly locked in if iOS has an app that $foo doesn't, but you are not locked in by having to pay $1 to re-buy it for the other platform.
People look at the phone + contract costs as fixed: "I'm going to have a contract with somebody" so if they have an investment in iPhone apps that they like they are going to be more likely to stay with iPhone/ATT then move to Android. Even if it only amounts to a few tens of dollars, there's also simple inertia at work here.
Having a button for the average user to press is what it's all about. "Press this button, and you can find apps." It just serves as a way for people to most quickly find something that meets their needs. For developers, it gives those people a place to look and hopefully increases exposure.