It lets them interact with other dogs they meet along the way. (pets.webmd.com)
She would probably be a GREAT running partner and does great walking on leash — but is not always friendly with other dogs she meets. (00520b4.netsolhost.com)
Sometimes they get over it as the puppy gets older, but if two adult dogs fight constantly it is unlikely that they are going to get over their aggression issues with other dogs met during a walk. (pethelpful.com)