Well, instead of writing this for every button in your web app:

<button class="bg-indigo-600 px-4 py-3 text-center text-sm font-semibold inline-block text-white cursor-pointer uppercase transition duration-200 ease-in-out rounded-md hover:bg-indigo-700 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-indigo-600 focus-visible:ring-offset-2 active:scale-95"> Button </button>

You can write this:

<button class="btn btn-primary"> Button </button>

The utility is pretty clear to me