If they did not modulate Mack the Knife the song would lose a lot.

The way I view modulation:
1-Start with a key that is a bit below my comfortable range.
2-The band is playing simple, very little happening.
3-With each modulation I get closer to the range my voice sounds best.
4-With each modulation the band becomes more exciting.

The finale product reaches a high point of excitement.

IMHO, John C.