What is the benefit of having baro-VNAV in addition to a WAAS GPS?
LPV and non-baro LNAV/VNAV requires that a Space-Based Augmentation System (SBAS) be available to the GPS receiver.  In the United States, that system is called Wide Area Augmentation System (WAAS).  Similar services operate in other parts of the world (e.g. EGNOS in Europe, MSAS over Japan, GAGAN over India) but coverage is not worldwide, and not all GNSS (Global Navigation Satellite System) approaches are approved for LPV.

Having a baro-VNAV system in the aircraft allows you to shoot LNAV/VNAV approaches if SBAS is not available. While not as useful in the United States, as we have robust and reliable coverage with WAAS, it is nice to have in parts of the world where SBAS is unreliable or non-existent.

FAA fact sheet on RNAV (GPS) approaches: http://goo.gl/cIhIJa

Worldwide SBAS information: http://goo.gl/eooOau