I agree on the uselessness of this. In fact it's not required in the URI spec.
I mentioned it because I felt it's widely supported that way. For example, I believe that in order to capture a link android using intent-filter it unfortunately assumes the "//": https://developer.android.com/guide/topics/manifest/data-element.
I rather have a clean and proper spec than to comply with some OS behavior. But cannot deny the practicality of it.
Thanks for NIP link!