It can be usefull to activate more than one call in or call out configuration for branches and customers at the same time.

And if you also set the dialmode for the activated configurations to auto,  you can have a seeming permanent networklink with those systems.

As long as there is no networktraffic between those systems and your Linux system,  there is no telephoneconnection thus there also is no telephonebill.

Only when you are accessing one of those systems,  there will be made a connection automatically.  This link will also be disconnected automatically,  if there has been no networktraffic over the link during some timespan.

Furthermore you can have more configurations active,  than the number of available telephonelines.  Of course it is not possible to have links with more computersystems at the same time than the number of available telephonelines.