chown which lets you change the owner and group of files and/or directories. But there's also
chgrp which only changes the group. Why was
chgrp created? Isn't it redundant?
chown initially couldn't set the group. Later, some implementations added it as
chown user.group, some as
chown user:group until it was eventually standardised (emphasis mine):
The 4.3 BSD method of specifying both owner and group was included in this volume of POSIX.1-2008 because:
There are cases where the desired end condition could not be achieved using the chgrp and chown (that only changed the user ID) utilities. (If the current owner is not a member of the desired group and the desired owner is not a member of the current group, the chown() function could fail unless both owner and group are changed at the same time.)
Even if they could be changed independently, in cases where both are being changed, there is a 100% performance penalty caused by being forced to invoke both utilities.
chown :group to only change the group is not portable or standard.
chown user: (to assign the primary group of the
user in the user database) is not standard either.
Root privileges are required when changing the user-owner* of a file.
A regular user can change the group-owner of a file to another group that he/she is a member of.
Maybe from this privilege perspective it made sense to keep them separate, especially in the pre-sudo era. Like
chown could have been made
setuid with a group of 'admin', allowing members of the 'admin' group to change the user-owner of files.
* I use "user-owner" and "group-owner" and "others" for consistency with ugo. Confusing "o" for "owner" is a common mistake.