In the standard workflow you have to option to assign a request to a person. The assigning can only be done for the current active step / stage you will not have the option to plan somebody in future stages of the workflow.
The standard assign also allows you to only assign to one person.
With the Workflow Plus Assign you can assign multiple persons to the current step in the workflow and to future steps in the workflow. This way you are able to plan the person(s) who while be handling the next steps.