Water is important in Islam. In order to perform salaah, enter a masjid, or hold a Qur'an, a Muslim must first perform a ritual cleansing called wudu. For convenience, Islamic holy sites, like Nizam al-Din's chilla-khana and khanqah have ablution tanks on site for use by those that intend to enter.
Some mosques, such as the Jama Masjid in Delhi, were designed with a central ablution tank that is sometimes called a fawwara.1
1. Philip Davies, The Penguins Guide to Indian Monuments, vol. 2, Islamic, Rajput, European (London: Penguin Books, 1989), 595.
O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles. And if you are in a state of janabah, then purify yourselves. But if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and do not find water, then seek clean earth and wipe over your faces and hands with it. Allah does not intend to make difficulty for you, but He intends to purify you and complete His favor upon you that you may be grateful.
The Qur'an 5:6.
The Delhi Walla. "City Faith: Hazrat Nizamuddin's Chilla Central Delhi."