tonality

I'm studying the tune There Will Never Be Another You and I have a question about the last eight bars aka B2 (the form is A-B1-A-B2). The changes are (in the alto key) | FMaj7 | Bb7 | CMaj7 | F#-7 B7 | CMaj7 B7 | E-7 A7 | D-7 G7 | CMaj7 (G7) | What I don't understand is the function of the Bb7...