Even on Apple's support document, it doesn't say what the benefit is. https://support.apple.com/en-us/108055
I understand why it makes sense to not fast charge the top end of the battery, but what are we saving by never charging above 80%?
You increase your battery’s useful lifespan, so can use your iPhone for longer with its original battery. If you are on a long (3-5) years upgrade cycle, this can be useful.