He doesn't attack the target he just grabbed half the time and instead go attack someone else on the other side of the screen. Very noticeable in congested late game fights.
Functionally, Blitzcrank will grab the target the further away from him. While I can see the appeal from a game design PoW, it makes his ability utterly useless. Any competent player will put a tank or another unimportant champion in the back. And the opportunity cost of having multiple blitzcrank on one team to "maybe" grab the main enemy ADC that you may not be able to kill anyway is not worth it.