After reading the Sikhnet response to the initial question, one's desire to understand the rationale behind the Hukkam Namma may not have been fulfilled. The rationale is simple. The Sikh Dharma prescribes a method for humanity toi elevate their Self to a higher state, and that is achieved through seva (selfless service)and simran (meditation). Guru Arjun desribes seva and simran as two wings that allows your Self to fly towards Waheguru. It is for this reason the Gurdwara has two halls, one for simran, the Darbar Sahib, and one for seva, the Langar Hall. The Darbar Sahib is the hub of all activity in the Sikh Dharma because the Guru Granth Sahib is the Eternal Spiritual Teacher for all humanity. It is from the the Guru Granth Sahib, the Shabad, that you and I have evolved from and are suppose to revolve around. That is, your soul has originated from the Shabad, the Naad that provides life to all, and your mind is to constantly meditate upon this divine Shabad & Naad (Life Force Vibration). The Lan
gar hall is the second component in a Gurdwara. The Langar hall should not be equated as been the same as a dinning hall in the Western Culture. Langar is a Sacred Meal that is prepared with love, devotion, and a meditative mind. Essentially, the Langar is Prasad, that is, the True Guru's Blessing. To experience the blessing, love, and devotion that is contained in the Langar, it must be consumed with a meditative mind. The meditative mind is aquired by sitting in the Lotus position (sitting on the ground cross legged and backbone erect). During the time of the Sikh Gurus, Sikhs would sit in the Lotus Position in the Langar and chant Satnam Waheguru with a meditative mind before Langar was served. Once all the Sikhs had gathered in the Langar and Satnam Waheguru was vibrating throughout the Langar, a Sevadar would say "Bole So Nihal" and the Sikhs would respond with "Sat Sri Akal".
A more broad reason for why Sikhs sit on the ground in a Gurdwara is to free the mind from the ego. The ego is the most pervasive obstacle to attaining Waheguru. In our regular routine, we enjoy the luxuries of life and usually aquire luxuries to elevate our status in society. Essentially, we become attached with these luxuries that we either have or desire to have. Such attachment makes us inevitably greedy for more and more luxury. However, obstacles arise in attaining these luxuries, thus making us angry. Such anger, greed, and attachment makes life unpleasant. To free our Self from such unpleasant feelings, the Guru prescribes seva and simran should be practiced with humility. Thus making sitting on the ground in the Gurdwara an act of Humility.