In this case it is intentional and basically became a feature to the point that probably majority uses.
The hash requirement is kind of a chicken & egg problem, because to fetch a file you need the hash, and to get the hash you need to fetch the file. So this gives a way to escape it.
There's also the "proper way" to use the prefetch command (I personally like the nix-universal-prefetch) which basically fetches file to the cache and outputs the hash, but in that case it is one extra command which you need to figure arguments for, so the former method is just faster.
The hash requirement is kind of a chicken & egg problem, because to fetch a file you need the hash, and to get the hash you need to fetch the file. So this gives a way to escape it.
There's also the "proper way" to use the prefetch command (I personally like the nix-universal-prefetch) which basically fetches file to the cache and outputs the hash, but in that case it is one extra command which you need to figure arguments for, so the former method is just faster.