There's a strong link between the latissimus dorsi muscle (your 'lats'), thoraco-lumbar fascia and opposite glute and hamstring muscles. The activation and use of these muscles and fascia are required to generate hip and leg extension, to propel the body forward.

Not using your arms to help contract and pull the hip and leg into extension, will reduce the potential power generated, making your running less efficient. It's not possible to run like a sprinter for more than a few minutes, but your arms can be used as a focal point when getting tired, in need of a burst of acceleration, going up an incline, doing intervals or pushing to the finish.

Here's the cue to focus on during your next run - to engage latissimus dorsi you need to actively pull the upper arm (humerus) back behind the trunk focussing on your elbow.  It's easy to increase tension in the neck, shoulders and arms when trying this so be mindful to keep the hands soft and relaxed and the elbows down by your side when moving your arms forward and backward.

Next time you're feeling heavy in the legs, or need to beat your running buddy, increase your arm's swing, focussing on the pull back through the elbows and feel the power.