There are two rain seasons in Kenya followed by a drought in-between. Heavy rain season lasts from April to June and a “short-shower” rain season lasts from October to December. All in between is a drought. So that is when the water is most needed.
Water tanks are installed under a house roof: when it rains, roof catches the water and water runs via roofs’ gutters into the water tanks. That means that water is within reach and people don’t make long “water-walks” that could otherwise take hours.  
Water accumulated into water tanks is a lifesaver during the drought. Rain water reservoirs help people to meet their daily needs of water consumption, hygiene and agricultural purposes. That provides sustainability for farming and food security for the families.