Bouncing the flash, whether on or off the camera, is a great help if I need to supplement ambient light to allow more depth in a shot, or where I need to even up a shadow with fill.
Direct flash tends to give a hard and flat light that would in many portrait shots lead to a subject’s characterful features being lost. It can also result in unsightly shadows in the background that draw attention away from the subject. Instead of simply pointing the flash directly at my subject, I find that bouncing the flash off different areas of a neutral-toned/white ceiling or wall gives me many more options in terms of the direction and quality of the light.
When I use a speedlight to capture a portrait I also bounce the flash to get softer shadows on my subject as this is often considered a more flattering way to light a subject’s features.
Remember to look out for neutral-toned/white surfaces to bounce off – a ceiling, a wall or a reflector are all great – and to experiment with direction of light to help capture subtle and characterful details.