The use of swales has been popularized as a  rainwater-harvesting and  soil-conservation strategy by Bill Mollison, David Holmgren, and other advocates of permaculture. In this context a swale is usually a water-harvesting ditch on contour, also called a contour bund.
Swales as used in permaculture are designed by permaculturalists to slow and capture runoff by spreading it horizontally across the landscape (along an elevation contour line), facilitating runoff infiltration into the soil.  This archetypal form of swale is a dug-out, sloped, often grassed or  "ditch" or "lull" in the landform. One option involves piling the soil onto a new bank on the still lower slope, in which case a bund or berm is formed, mitigating the natural (and often  hardscape-increased) risks to slopes below and to any linked watercourse from flash flooding. 
In arid and seasonally dry places, vegetation (existing or planted) in the swale benefits heavily from the concentration of runoff. Trees and shrubs along the swale can provide shade and mulch which decrease evaporation.