For some organisation, it's difficult to engage external users / partners in the auto-registration process.
It takes to much consuming time and effort to manage credentials. Pasword lost, long-time registration, captcha...
So it could be a cool feature, if internal users can send one time link usage for downloading files.
The user will receive an OTP link by mail to get access to the shared file. Each time the user want to get the shared file, he has to click the OTP link validating by his email adresse and get access to the file. So it can't be someone else !.
As exemple :