Ah. I want to set it b/c I want to run the following to clone the roles defined in my current playbook's requirements.yml file in a roles sub-directory relative to my playbook.
$ ansible-galaxy install -r requirements.yml
So if my playbook is in....
/home/myuser/ansible-plays/myplaybook.yml
...and I run above command in that dir, I want the roles to be in
/home/myuser/ansible-plays/roles
Thanks again