The episode is a Sabbath controversy, and is found also in Mark 2,23-28 and Matthew 12,1-8. Since Deut 23,24-25 allowed a person passing a neighbour’s field to pluck heads of grain with the hand, this does not seem to be the reason for the complaint of the Pharisees. Luke (6,1) alone states that the disciples were rubbing the heads of grain in their hands, which could be interpreted as threshing, and threshing was one of the forms of work forbidden on the Sabbath. In his response, Jesus refers to the incident from 1 Samuel 21,1-6 in which David confronted Ahimelech at Nob. The point that the Lucan Jesus makes is that if David had the authority to overturn Levitical rules and eat the bread of the Presence and even give it to his companions, because he gave priority to human need over ritual observance, so can Jesus, who is Lord of the Sabbath.
Rules and regulations are made so that there might be order in society and each will know his/her role. It is possible that sometimes they might become ends in themselves and take precedence over everything else. They can never take precedence over human need. All rules and regulations are at the service of humans and not the other way around.