Ux CSS Lib is build upon Bootstrap 5.0.
It's meant to be used with ux projects so it is built to work with fast prototyping philosophy.
When you are fast prototyping a project you don't want inside all details of your css, maybe you just need a top-level class that describe your component.
For example you want to do something like this:
div
.className sidebar
ul
li 'item 1
li 'item ...
Instead of:
div
.className sidebar
ul
.className sidebar-group
li 'item 1
.className sidebar-item
li 'item ...
.className sidebar-item
So you rely on the structure of your html.