I have an iPhone 5s. I wonder if using an additional lens increases the image quality or not. Or does it just help in zooming?
It could, in the case where you would otherwise use the "digital zoom" provided by the camera interface, which really just crops the image, and always results in lower quality. If the extra lens is low quality, though, you could just be trading one kind of image degradation for another.
Also, depending on what you mean by "image quality", a polarizing filter might also be considered to increase it in some cases.
Your sensor is going to make a big determination of what your picture will come out as.
If you have a junk sensor then lenses won't help.
If your first lens is scratched, then you're still going to have a scratch.
However, if I have a cheap lens and a good lens on the same body then I'll only see a quality difference if the sensor could detect those differences.
Also, as stated before, an optical zoom will produce a higher quality image than a digital zoom; however, I don't think that is the question you were asking.