I am, by nature, a tinkerer. When putting a design together rarely am I satisfied on my first attempt. Rarely, am I satisfied with my design until iteration 3, 4 or even 5. Let's see how a limited set of choices can help you create a cleaner page with less CSS and less mental overhead for designers and developers.
Tailwindcss has a well-crafted design system of utility classes. If you are unaware of utility-first CSS it is a system of hyper-focused classes that do exactly 1 thing.
shadow-lg are responsible for doing 1 thing, increasing the horizontal margin or adding a large box-shadow. These classes can be combined to create cards or buttons or any other aspect of your page.
Utility classes provide the guidelines to your design system. The design system allows you to tinker by changing values but only to a limited set of alternatives. The creators of Tailwindcss like to express this as a "constrained set of choices."
A constrained set of choices limits your ability to adjust the margin from 1rem to 1.05rem as that is not a choice within the design system provided. Why is that good? If you need 1.05rem shouldn't you be able to use that value as your margin? Seldom has such fine-grained control over such small details lead to a better design. You've introduced a small detail that may only appear in 1 place, or worse still, may need to be remembered and used in other places.
Instead, Tailwindcss provides a thoughtful design system of limited choices. The choices provided are flexible but still force you to work within the lines.
One key to a professional-looking design is consistency. Having to decide about spacing or shadows or font sizes while trying to build a new design will add a lot of mental overhead. Tailwindcss takes those decisions out of your hands and has created a design system that allows you to be flexible, tinker.
The limited choices available helps both designers and developers be more consistent and agree on a design language without needing a full blown design system.
Having a guide such as Tailwindcss will always reduce your CSS bundle size. Taking the choices out of designing will prevent designers and developers from adding 9 versions of red, all which look the same. Or, adding 12 different box-shadow styles where 4 would do.
It is a dual benefit, reduce cognitive overhead of making design system choices and also prevent CSS bloat. When there are only 4 well-known choices for a box-shadow it is harder for developers and designers to go rogue and introduce their own. Trust and use the system.
The constrained design system isn't unique to Tailwindcss. A similar system could be implemented using SASS variables. This would be a workable alternative but would need an immense amount of work to recreate. Additionally this is something that Tailwindcss has already done very well and provides a ton of flexibility with the configuration system.
Get out there and start coloring.