Because it provides a copy-pastable subset of reliable classes where you end result will look good.
As a company we are often hired to fill a knowledge gap (exactly in design and front-end). The nature of agency work is to leave a deliverable for the client to work with.
My idea is that when the project is over and the design/front-end gap still exists in the team, perhaps it is better to leave something more manipulatable (I used the word malleable originally).
I think with the great docs that Tailwind has it might be easier for someone who is not a front-end dev to manipulate a `<div class="p-4">` to `<div class="p-3">` than to come across a BEM/ITCSS component, written in SCSS* where you have to understand much more concepts to manipulate it skillfully.
*(our preferred stack really)