It is useful to use a Group-Picker in connection with the Collective Issue View Plugin, if every user that creates a ticket is member of one group rather than several groups. Then the (double) configuration of unites is not necessary. One scenario could be that you run a service desk for business customers and you have created a group for each of your customers, where all employees of the customer are members.

You can use the Participant Unit Picker Custom Fields for cases that reflect more complex scenarios.
You can combine several groups in units and thus simplify the choice for your users.
You have the option to automatically set participants for some units when creating a ticket. For other units you can choose that only after manually selecting the unit, additional participants will be added to the ticket.
You can define different groups for the units which will trigger setting the participants and who will be set as participants. For Example you might want to specify to inform the complete team if a ticket is created by management. At the same time, management would not be informed every time, the team creates a ticket.

Yes it is. For units that you don't want the participants to be set automatically, just leave out the trigger groups when creating the issue. You may then select the unit manually as a Service Desk Agent. When manually selecting the unit, the participants are set from the viewer groups.
Additionally, you can also set the units via post function (see next question), i.e. if you choose to check the issue before informing other participants.

Currently this is not supported, since the Participant Unit Picker Custom Field is not supported by Service Desk. In JIRA you have the opportunity to manually select a a unit (see previous question).

No this is not possible. The Collective Issue View Plugin currently supports only the simultaneous use of exactly one Unit Picker Custom Field per JIRA instance. A Unit Picker Custom Field per project can not be configured.

Yes, they can. For setting the unit you can use the post function "Update Issue Custom Field" of the free Suite Utilities for JIRA plugin. In order to set a unit via post function in the Participant Unit Picker Custom Field, you need the ID of the unit.
You will find the ID of the Unit in the configuration of the Participant Unit Picker Custom Field.

For example, to set Unit Team B, you have to use ID 12 in the post function.