Aren’t the Qur’an and Sunnah enough, why do we even need Tasawwuf?

The superiority of the Qur'an and Sunnah is not in question. Tasawwuf is the practice of applying them, since knowledge is not sufficient in itself. Knowledge of the Qur'an and Sunnah does not suffice until it is lived.

The superiority of the Qur'an and Sunnah is not what is at stake here. Rather, the essence of all substantive prescriptions of the Qur'an and Sunnah lies in human attitude toward the Divine, and knowledge does not suffice until it is applied.

For example, if a non-Muslim recited the entire Qur'an from beginning to end, it would not benefit him, yet reciting the Shahada once, with conviction, would. He is standing before a door, and needs the key of iman. Tasawwuf is the discipline of taking the knowledge one has and turning it into a lived state, under the guidance of those who possess that knowledge themselves.