Preaching is the duty of sannyasis
Preaching is the Service that pleases Lord the most. For one who explains the supreme secret to the devotees, devotional service is guaranteed, and at the end he will come back to Me. Bg 18.68 But we have distinctions for the proper functioning of the society, Varnasrama dharma. If everyone wants to engage only in preaching Service then who is there to take up other services? all other services requires to be rendered for Krsna. In order to build a organized society, we need to introduce VAD. Different persons have different priorities in devotional service. Srila Prabhupada explains that For paramahaṁsas, or sannyāsīs in the Vaiṣṇava order, preaching is the first duty. Srimad Bhagavatam 7.13.9 For a brahmacari, the first duty is to serve the Spiritual master and learn the vedic knowledge. For a Grhastha, maintaining family, charity, producing children, deity worship, yajna For a vanaprastha, austerities and p...