Usually, a clip-on phone lens does not magically improve the camera’s overall image quality. The phone’s sensor and built-in lens still set most of the quality limits.

Where an add-on lens can help is by changing the optics before the image reaches the sensor. For example, if you would otherwise use digital zoom, an optical add-on lens can give better results because digital zoom is essentially just cropping the image. That said, if the accessory lens is low quality, it may introduce softness, distortion, or other degradation, so you may simply trade one problem for another.

In short: a good add-on lens can be useful for reach or a different field of view, and it can beat digital zoom. But it won’t generally improve the camera beyond the limits of the phone’s sensor, and a poor-quality add-on can make images worse. Filters such as a polarizer can also improve certain images by reducing reflections or glare, depending on the scene.