[jlocdog]

Obviously you haven't read what I have said or atleast what is implied. He is mixing up his play to
a)make his c-bets less exploitable,
b)induce bets from hands that would normally check,
c)give off a unorthodox image of play which in turn makes it hard to get bluffed/thin valued,
d)have more control over the size of the pots you enter because your c-bet % will now decrease if you so choose,
e)allow for more aggression on later streets because of these smaller pots
f)enable you to utilize the donk if you so choose because of your now unorthodox image,
g)blah blah blah....

No one is jut 'mixing up their play' because a book says to do so. There are obvious reasons one would do so. The examples I gave are just a few in which it may be a good time for it. When the hand is in your favor is always best rather then trying to manufacture a play just for the sake of mixing it up.