Edit: I eat my words below because I didn't know what I was talking about. Thanks for pointing out bruceboughton and mbarr.
<uninformed>The "send to phone" button on app store webpages is a simple feature Google and Apple can copy straight from Amazon's Kindle playbook. If I couldn't send book samples to my Kindle while browsing Amazon on my computer, I'd miss out on checking out and buying a lot of books.</uninformed>
This is part of iCloud isn't it? Anything you purchase is synced to your devices, so you can purchase in the store on your desktop/laptop and it is synced to your phone/iPad/iPod.
<uninformed>The "send to phone" button on app store webpages is a simple feature Google and Apple can copy straight from Amazon's Kindle playbook. If I couldn't send book samples to my Kindle while browsing Amazon on my computer, I'd miss out on checking out and buying a lot of books.</uninformed>