Bowen uses such light pressure because it follows a very specific physiologic law of the body, the Arndt-Schulze law, that states the body responds more profoundly and positively to a low level stimulus than it does to a strong painful stimulus. Less really is more in the case of Bowen. It uses the least amount of work in search of achieving the greatest effect approach.
Also, the art of Bowen is for the Practitioner to apply enough pressure to address the body physically and at the same time to apply the pressure light enough to address the body energetically at the same time!