in my opinion it is the events and state transitions that are where everything interesting and challenging happens, like server IO in mount/unmount/didUpdate lifecycle methods or event callbacks. For example, a typeahead component. You focus the input and start typing. The picklist unfolds and loads records from the server, we need to stream subsequent keystrokes to the server into the query for server filtering, we need to maintain the DOM gracefully as the query changes stream in, we need virtual scroll if the collection is large, the user then uses the arrow keys to select an item in the picklist and presses enter to submit, closing the picklist and returning the selection to the caller. It is a living, reactive process. Now "use the REPL" to get, like, any value at all?