Sites often have sets of users that have the same job or role. You can have multiple CAD specialists working on updating plans, for instance. You might run a series of outsource or contract people through that role as well.
From a security standpoint, roles define how the generic groups map to the needs of classes of users within your specific organization. As such, you don’t assign security groups directly to users, but instead you assign roles so as to give the appropriate access to all relevant groups all at once.