You have an entirely different onboarding story on the web. You can test easily, cheaply, and fast enough to make a difference on the web. You can fix a critical bug that crashes your app on load 15 minutes after discovery. You can show 10 different landing pages and decide in real-time which one is working the best for a particular user. You can also close a viral loop: A user can click an email and immediately be using your app with you.
Vibhu Norby, Why We’re Pivoting from Mobile First to Web First