The use of ismemberof is to offer the chance to anyone to access a dashboard but data will be restricted to the people from that group.
Let's say you have 2 views in the dashboard, one with restricted data and one with general data. That will enable anybody to see the general data but the other one will be restricted. User group permissions will restrict the views / dashboards entirely.
You can find more information in the links below
Thanks Ciaran for your valuable input.