Calling logout explicitly may help if you want to avoid a user automatically using another login access pass when their existing login access pass expires. However, force logout does not necessarily save a token. I see force logout as a method to use if you have Qlik Sense embedded in host application and you want to terminate the Qlik Sense application when the user logs out of the host application.
Keep in mind that if you force logout and the user logs back in after their previous login access pass expires, they will use a new access pass. In addition, remember that a login access token contains 10 login access passes. Each pass has an hour of time attached to it that expires one hour after the pass is granted.