Here is a very common usecase:

in a deployment script.

I have always used "sudo" with this because I didn't know why I was getting an operation permitted error when doing so. Until I found out that if the effective user is a member of the target group www-data, the sudo isn't needed.

The Wikipedia clearly says that:

The chgrp (from change group) command may be used by unprivileged users on Unix-like systems to change the group associated with a file system object (such as a file, directory, or link) to one of which they are a member.
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Wouter Thielen