The Coordinate enables its holder to control the Titans. It has been observed that regular Titans simply obeyed the orders given by its holder while people with Titan powers can sense its use, though they are seemingly not influenced by its control.
Before fleeing the Scouts in the Forest of Giant Trees, Reiner states that the connections between the Reiss family and the Order of the Walls make Historia Reiss a valuable asset for the warriors' mission, and having her with them will make their goal much easier if Eren Jaeger is not the "Coordinate" they are searching for.
Later that day, the warriors' suspicions are confirmed when Eren unleashes the Coordinate, striking the hand of a smiling Titan and causing all the surrounding Titans to swarm it and tear it to pieces. When Reiner and Bertholdt approach him afterwards, Eren's outburst once again triggers the Coordinate and sends the nearby Titans rushing towards the warriors, allowing for the Scouts to escape. On the journey back to Wall Rose, all Titans encountered ignore the Scouts and instead run in the warriors' direction under Eren's subconscious orders.