Are the designers build the UI kit for the designers?

There are many design systems available online noways, but I am getting confused - what was the purpose of buying a designer system for the designers. 

Why designers buy the UI kit instead of making or building their UI kit? How can it be helpful for the organization and designers? Is it help the UX designer to focus more on the solution instead of crafting the interface?

If you ever buy any design system, please share your experience.