Essentially, the image quality. Image quality (sharpness, contrast, colour fringing, etc.) cannot be read off from the given key figures. I used to have the old "non-L" lens and because it was not satisfactory by my standards, I bought this one and am more than satisfied. But it depends on your own standards, of course the consumer lenses (not L) are better than a smartphone. For quantitative measurements, the site https://www.dxomark.com is recommended.

Furthermore, the L lenses are dust and splash-proof and are much more robust and durable, i.e. for professional use.