Because maybe you use S3, which treats `foo/bar.txt` and `foo//bar.txt` as entirely separate things. Because to S3, directories don't exist and those are literally the exact names of the keys under which data is stored.
So you have script A concatenate "foo" + "/bar" and script B concatenate "foo/" + "/bar", and suddenly you have a weird problem.
I can't imagine a real use case where you'd think this is desirable.
Not S3, but here's a literal real use case: the entry for the Iraqw word /ameeni (woman) in Wiktionary.
https://en.wiktionary.org/wiki//ameeni
If for whatever reason your S3 keys contained English words and their translations separated by a slash, you would have a real problem if one of your scripts were to concatenate woman, / and /ameeni as woman/ameeni instead of woman//ameeni in the English/Iraqw case.
woman/%2Fameeni
ameeni//ameeni
W3C says:
> The slash ("/", ASCII 2F hex) character is reserved for the delimiting of substrings whose relationship is hierarchical.
Can they not just use a 3 like in Arabic?
On S3 they're different. Using the wrong paths causes weird issues, like not finding things you expect you find, or storing multiple versions of the same data out of sync.
Normalizing // to / means making S3 behave more like people expect.