¿Cómo estás tú? vs. ¿Cómo estás?

What's the difference between using "¿Cómo estás tú?" and "¿Cómo estás?"

They are the same. You would use the first version to emphasize YOU. Otherwise, it is completely clear and acceptable to use the second version that you have listed. The two are practically interchangeable.

I see. Thanks for your reply :)

Spanish verb conjugation is often so precise that the pronoun is not needed, unless you really want to place emphasis on the person (at least in in first and second persons).