On the jQuery UI site, you can create a custom build for your own purposes depending on what components and features you actually need. So if you just need a modal pop up window, you can include just that feature and not have the bloat of the rest of the package. While there are front-end frameworks that are slowly introducing some of the components that are found in jQuery, the majority of them are not. For a website or app that needs to include a modern user interface, jQuery UI is a useful tool.

It makes it easy to add useful UI components like date pickers and sliders, without needing to implement a lot of custom logic.

Its easy to get some advanced UI components up and running very quickly. Theres not a ton that you need to learn to get started with jquery ui.