Intensity in Single-Slit Diffraction

Source: https://openstax.org/books/university-physics-volume-3/pages/4-2-intensity-in-single-slit-diffraction

Why is there a $2pi$ in the numerator of the phase difference equation? I understand that we want the phase shift in radians/degrees, but isn’t it already in radians by the $sin(theta)?$ I tried watching ilectureonline videos for this, but it did not really clear up my question.

ilecture videos: https://www.youtube.com/watch?v=OuaPzAN67fw, https://www.youtube.com/watch?v=9b4HYt6F53Y&t=433s