Course AWS - IAM / Lab 02 Question

I’m on lab 02 on the course AWS - IAM/ Amin Mansouri.
we are asked to create a policy for certain groups, the solution provided is to create a policy and then attach it to the group.

it is not clear why “Resource Based Policy” as demonstrated in one of the session is not valid solution and when you need to use Resource Based Policies, with the “Principal” pointing to the IAM group.