SP2 just lets you add a second service provider entry.
You could use a second Vestalink number (e.g. a second DID number, giving you a "line 2") or another provider (CallCentric, etc.) if you want some other feature or calling rate that they provide that Vestalink does not, for example.
The use of SP2 is totally optional. Most people probably use their ObiHAI with a single provider because that meets their needs.