The whole editorial and production process becomes continuous and the concept of editions goes away. WebTexts have an advantage over ordinary text books because they can be interactive. You should be able to ask the WebText author questions of clarification. You would like to be able to communicate with other teachers who are using the WebText and you would like your students to communicate with other students who are using the WebText. You can integrate student activities into the WebText.
Obvious things are interactive quizzes and assignments.
Students can add their own ideas and research to the WebText. Creating a WebText and keeping it going is a non-trivial task and requires an enormous amount of initial and ongoing effort. It is not a part-time activity for an overworked teacher. We need to invent ways in which authors can be paid to create and maintain WebTexts. This may evolve through existing publishers of textbooks although if history is any guide it is likely that the textbook manufacturers will be unable to move to the production of WebTexts.