To make our self safe from the action of inertia of motion.
When we are in a moving object we are also moving with the same speed relative to the ground.
So to decrease the inertia of motion we run in the direction of the moving object. :) so that our foot doesn't come in rest as soon as we land on earth .
Cause if that happens we will fall on the ground.
As our upper part will still be in motion.
To overcome the inertia we run in the direction of motion. :)