Bottom corner seals (In Chinese yajiaozhang 押角章) are just as the name implies—seals placed to mark the corners of the finished artwork. These are especially useful for a work of art done on a large piece of paper that will be cropped before mounting, matting, or framing. The seal indicates to someone other than the artist where the artwork is meant to end.
The illustration is a landscape painting by Huang Binhong (1865-1955) showing a bottom corner seal.