What is the purpose of the cowl vent?

[bigoilman]

Thanks. I have Vintage Air in my 1936 Chevy so I'm familiar with that type of cowl vent. On theses trucks it just seems like a poor design. As I understand it, the A/C on "max" recirculates air instead of taking in new air. Previous owner of my truck said the area was covered by sheetmetal by the original owner. Presumably because of a leak. The guy I bought it from couldn't make it leak (he tried), removed the homemade cover and installed a new stock grating. He may have fixed the presumed leak with the new windshield and seals before trying to find a leak. Just wanted to know if it needs to be open for normal operation of HVAC system.