Between "it opens on my computer" and "there's a link other people can open" lies the step where most people give up. Only after publishing do you hit the real problems: buttons too small on a phone, a blank page for someone else, data gone after refresh. None of these ever show up on your own computer.

More importantly, a link is something you can show anyone. Week 2's mini app is the same: when it's done it runs on a phone, not in a simulator.