For a perspective on websites that move from “website” to “web app” start by looking at a site like Zappos that is mostly marketing but has some information sorting. Then a site like Stripe has a marketing side on their main site, but an almost entirely informational section for managers and a very focused API section in particular. Last a site like Trello disappears once you’re logged in, the marketing is done, and the entire app runs within a browser.
Pay attention to what elements remain, what change, what stay the same between the different examples and their lifecycles.
Online Shoes, Clothing, Free Shipping and Returns | Zappos.com
Stripe – Online payment processing for internet businesses
Stripe API Reference