What is a UI Kit, how to use them, what is the profit from them?

Question

User interface

I often come across such a product category as UI Kit. Example - https://ui8.net/categories/ui-kits
How and by whom is it used in practice at work?
This is purely for designers, or what? Not for coders? What is it in general? A set of pictures? Project in PSD? Ready code?
If these are just pictures - then why the hell does anyone need it at all?

Answer 1

How and by whom is it used in practice at work?

Designers can buy to make work cheaper. Also, customers themselves or layout designers can buy yuaykit in order to use it in a project that needs a little more than standard bootstrap elements.
In general, this is approximately a project in the PSD.
It can be used even at the prototyping stage.
Coders who can chop the PSD, or build a website with PSD can also use this.
If standard bootstrap elements are enough for your projects, you don't need them. If you want more customization, but do not pay for custom design - yuaykit. If you have hired a designer for custom design, but in the process you are adding application functionality, your developer will need a yuaykit. This is a kind of brand book for your application.