In Buddha’s time, monastics did not have a fixed abode. This means that lay buddhist could not easily give support or learn from the Buddha or Maha Sangha. Lay buddhists would, in time offer groves, gardens or parks to the Maha Sangha to reside so that the lay people have the opportunity to give their support and learn from the Buddha and the Maha Sangha. This allowed the Four-fold community, ie. Bhikshu*, Bhikshuni*, Upasaka and Upasika (Monks, nuns, lay men and women), to grow together healthily.
Even with some lay people being enlightened, they never ventured into establishing a standalone lay community isolated from the Maha Sangha. Perhaps it is precisely because these lay people were enlightened, that is why they did not seek to establish a separate sasana from the Maha Sangha. Rather, they became even more ardent and earnest in their support of the Maha Sangha as they realise that the monastic life as instituted by the Buddha provided the best avenue for anyone willing to take up the robes, to practise the Noble Eightfold Path and attain Nirvana.
The Maha Sangha likewise was not to become isolated from the lay community. It was advised by the Buddha to neither neglect the spiritual needs of the lay people nor become inexplicably intertwined with them. The sangha if it became isolated from the lay, would lead to a decrease in the learning and practise of Buddha-Dharma in the lay, and over time the demise of the Sangha. If the lay isolates itself from the Sangha, it too cannot claim to be a complete four-fold community and without the Maha Sangha, whenceforth comes the Triple refuge?
Footnote:
Bhikshu (Sanskrit) – Pali: Bhikkhu
Bhikshuni (Sanskrit) – Pali: Bhikkhuni
EDIT: Thanks to a Venerable, Lay man and woman spelling in Pali is corrected. 🙂