I thought it was to reduce backpressure.
Most windshields attempt to direct airflow over the drivers head, and doing so creates a low pressure vortex behind the shield. you might experience this as wind pressure against your back as air tries to fill the low pressure area.
The slot attempts to allow a small amount of air to reduce the amount of back pressure and any buffeting caused by it. The riding position on the XC is naturally farther back from the windshield, and can leave a rider sitting in that vortex.
Not an expert, just an observation.
Every day is a good day to ride. Some days are just better than others.
2010 Cross Country
Vic Performance filter, D&D, ATW, VFC III
2006 Vegas (sold)
2003 Shadow 1100C (sold)
Various other bikes all the way back to 1968.