Why should I need multiple ViewVC instances?

If you have multiple Plone sites on the same Zope instance you may want to configure different cvs_roots for each Plone site.
With ViewVC this would mean setting up a ViewVC instance for every Plone site, whereas ViewVC++ lets you configure multiple "sites" so that every Plone site is associated with a ViewVC++ site and only a single ViewVC++ instance is needed.