It provides the opportunity to inject a shot of bold colour without it being overwhelming. It's also another way of repeating an accent colour within the general scheme, linking the window area to the rest of the room.
The use of Vertical or horizontal edging can be used to add visual height or width to a space.
Contrast edging can be created by applying a wide braid or by using a different coloured fabric, which gives the option of a much deep border. A deep darker horizontal border at the base of curtains 'grounds' the paler fabric and adds a touch of the unexpected.