There are a myriad of examples anyone can come up with that require a UI library to simply be fast. For example, perhaps someone wants to implement minesweeper with checkboxes. Or build an "infinite" feed, or a settings panel showing thousands of settings of a system where you can filter with a filter box, etc., etc. You can argue against all of these cases, or you can simply rely on a fast UI library and be done with it.
There are a myriad of examples anyone can come up with that require a UI library to simply be fast. For example, perhaps someone wants to implement minesweeper with checkboxes. Or build an "infinite" feed, or a settings panel showing thousands of settings of a system where you can filter with a filter box, etc., etc. You can argue against all of these cases, or you can simply rely on a fast UI library and be done with it.