Drain Holes on Dash Inside Windshield

[DJ_Baar]

I remember prior Owner telling me he put silicon in leaks under dash - I found the three (3) factory holes he filled when restoring my '67 Chevy C20. See picture, this is one I found on passenger side. There is same on driver side and in middle of dash at windshield had covered by windshield seal.

* Can some one explain the WHY and won't any moisture just go into wiring and things under dash?

R/
DJ