Tailwind, by intention, is aiming to equip you with an arsenal of utility classes by generating more than you need. There are some best practices which will help keep this overall build size down, like limiting your colors and breakpoints or turning off the modules by default before adding them as necessary. Still, you’ll inevitably generate classes that go unused. And honestly, approaching your configuration with an unrelenting miserly attitude will slow you down and make development less fun. By leaning on Purgecss, there’s no worry that the CSS your users download will only include classes that are ultimately needed.
I’ve never used Tailwind or Purgecss, but I reckon the latter could be particularly useful if you have a giant old codebase and you don’t have the resources to refactor things just yet. I guess my only concern with introducing a tool like that is it could encourage folks to not refactor large and problematic areas in their styles – taking the safest route with this tool instead.