The Mac App Store was a minor revolution when it first started doing business back in 2010. Like the iOS App Store, it lets you easily buy, download, and update apps that you can use on multiple Macs. It also serves as a conduit for updates to Apple software: both the operating system and Apple's apps, such as iTunes, Pages, Xcode, and others.
For many users, it's a great way to buy apps. It offers a single location to get software, and you pay Apple using your on-file credit card or gift card balance. You don't need to worry about giving your credentials to an unfamiliar website, you don't need to store serial numbers, and all your updates come through a single conduit.