Speaking anecdotally, up to around 2016 every other website used Bootstrap or Foundation, often with minimal customization, just because it looks and works well out of the box on all devices. Now it seems like it's getting better, but generic UI components are an extremely relevant aspect.

Promethease is a good example. Years later a bootstrap website signals to me "it's about the functionality, not the design". https://promethease.com/

Before that everything was jQuery UI. Government websites still use a ton of it. All of Colorado's web portals are built with it. And now things are shifting towards UI frameworks "plus more", like Ionic, which adds a bunch of design/devops workflow and cross platform functionality.