Because they already know the score.
A Boomerang is someone who worked at your company before, left, and wants to return.
Jenny Klimas argues that companies should make a special effort to attract them. Why? Because they already know everything about the firm and you know everything about them.
She claims that they are going to be very loyal after having discovered that the grass is not greener on the other side. But, of course, this might simply be a marriage of convenience.