1. Ads/trackers/etc. need javascript 2. It's a way of flexing and saying "we have resources to put into this webpage which makes us a serious business."
Any other thoughts?
- Toggling widgets such as menus, modals and other things you only want to show when the user requests it. This includes updating accessibility related HTML attributes.
- filtering, sorting etc. of larger data sets in the client.
- live updates of fresh, time related data
- search that doesn't force a complete reload, via AJAX or cached on the client.
- smoother page / content transitions via AJAX
- everything related to forms / user input: you want to instantly react
- managing and preserving state / context per user
- visualizations / graphs that are explorable / interactive
- polyfills for older browsers that don't support optimizations such as lazy loading.
- interactive widgets such as chat boxes (not a fan but still)
- testing and analytics
A website isn't made of paper.
Two other possible reasons from the top of my head:
If a web developer is hired to make a site they can probably charge more if it is a fancy javascript site. In some cases it might be in their self interest to up-sell this to a client that does not know better.
If a web developer makes a site for themselves I am sure many want to take the opportunity to get some experience in the latest web-tech while they are at it. Just as I will use an obscure programming language for my next side project..