If we’re playing with a band, we would need a general consensus and values firmly stated down, as we can’t have everyone changing chords on different values.
If everything were to be straight, changing chords on “1” and “3” for each bar, wouldn’t that be boring? Hence we are going to learn different counts for us to change chords on.
*Take note the second half of the structure in (b) is different, it’s normally done to lead into another part of the song, eg the chorus.