What is GroupId used for in Create Role API?

I am using the Create Role REST API to better manager group memberships. 

The API makes reference to a GroupId optional value, which I assume is the GroupId of a Group. 

 Create Role REST Endpoint 

Request Parameters

Name Type Description Required Default Options
Name string Name Required
Description string Description Required
GroupId int? Group Id Optional
UserId int? Either UserId or Username is required when using include=user Optional
Username string Either UserId or Username is required when using include=user Optional
RoleId int? Either RoleId or RoleName is required when using include=user Optional
RoleName string Either RoleId or RoleName is required when using include=user Optional
Include string Options for include are: user. Ex: include=user. Optional

However, I am unclear about what adding that value does. It doesn't seem to add the Role as a member of the Group and I do not see a difference when inspecting the role after creation.

Could someone clarify that this Optional parameter does?