The main use of the address book is to support the selection of people for invitations to workspaces, appointments and voting: for this purpose, the 'Select from my address book' option is always offered in the respective action form.
You can contact the people represented by the contacts of your address book, provided that email addresses, cell phone numbers or other contact data are available. This works the same way as in normal contact lists and is described in 6.1.5 Contact contacts.