Here's a breakdown:
How it's supposed to work:
* Spreads the light: The diffuser aims to spread the harsh, direct light from the flash over a slightly larger area.
* Softens the light (slightly): By spreading the light, it can theoretically reduce harsh shadows and hot spots.
Why it often doesn't help much:
* Too Small: The diffuser is simply too small to make a significant difference. The light source is still essentially coming from a point, just slightly larger.
* Limited Coverage: It doesn't spread the light wide enough to truly soften shadows effectively, especially in larger rooms or with multiple subjects.
* Still Direct: The light is still pointing directly at the subject, which creates the telltale flat, "deer in headlights" look of direct flash.
* Red Eye: It often doesn't do enough to eliminate red-eye.
When it *might* help a little:
* Close-up Portraits: In very close-up portraits (think headshots), it might soften the light *slightly* compared to no diffuser at all.
* Macro Photography: When shooting very small objects, a small amount of diffusion can be beneficial.
* Nothing Else Available: If it's the *only* option you have and the light is *extremely* harsh, it might provide a marginal improvement.
Better Alternatives:
* Bouncing the Flash: This is the *best* option if possible. Aim the flash at a nearby wall or ceiling to create a much larger, softer light source. This significantly reduces shadows and creates a more natural look.
* External Flash with Diffuser: An external flash unit is more powerful and often allows for tilting and swiveling the flash head for bouncing. Aftermarket diffusers for external flashes are also often larger and more effective.
* Off-Camera Flash: Moving the flash off the camera axis provides better control over light direction and shadow placement.
* Reflectors: Reflectors can bounce ambient light or flash fill in shadows.
* Natural Light: Whenever possible, use natural light. It's almost always superior to on-camera flash.
In Summary:
While the built-in flip-up diffuser isn't entirely useless, it's a *very* minor improvement over direct flash. It's usually not worth relying on for consistently good results. You're almost always better off exploring alternative lighting techniques like bouncing the flash or using natural light. Think of it as a tiny band-aid for a major lighting problem.