[whstl]

Projects that don't need a lot of interactivity after rendering definitely don't need one and you can get away with rendering everything in the backend.

Other projects need some additional features that must be implemented in the frontend, but still don't need more than vanilla JS or jQuery.

Others might need more complex components, such as datepickers, carousels, interactive charts, interactive tables, accordions. But even those can be consumed from third-party components without a framework. A middle ground is writing your own encapsulated components.

However there are more complex apps that do benefit from frameworks. It's often because they have a lot of custom components and a framework really helps; and/or because they're not really divided into pages in a traditional web way, so rendering on the backend is significantly harder; and/or they have a lot of shared state between multiple areas of the screen, and not refreshing is easy than caching or re-fetching. All those among other reasons. Slack Web can benefit from this. Your daily CRUD not so much.

Whether people are using the right tools for each job is up for debate. And sometimes you'll have incorrect requirements. But there are definitely reasons to use more complex/flexible tools.