A Swale is a ditch on the contour. It does not direct water, but holds it and allows it to gradually infiltrate the soil down-slope of it. Soil and water run-off are caught in the swale which becomes a fertile area. Gradual infiltration of water and nutrients and the dead roots of plants growing in the swale, slowly improve soil structure down-slope.
Swale is a land form that is down instead of up. In landscape, it is distinguished from a ditch by being less distinct. A swale is a gradual depression, much wider than deep. The landscape use of swales is indeed to carry water as drainage, but not so as to erode its bottom and so not requiring any different materials.
The test for a swale is whether you could easily mow across its width, because there is no sharp change in slope angle from the rest of the surface nor especially at the bottom.