The rationale is so that users can easily run something like "sudo
leafpad" or some other GUI, and not need to remember the right "su"
flags variables to make it work.
I'm not a fan of letting users run sudo with their own password by
default. You are welcome to use it, but I feel by default it is a
unnecessary security risk. I have systems here where the user needs
their PW to login, but I don't want them to have root access. Again, its
just a default though, so if you want something different, feel free to