Well the main purpose is to allow fresh air into the passenger comparment mainly for heat. The newer cars can circulate air within the passenger compartment but older cars cant because they are ducted directly to the outside air vents. Another purpose is to channel rainwater out past the vent ducts going into the car and dumps it inside the fender and out onto the ground. You can block it all off but you would have no air for heat which if you only drive in the summer is ok. But if you do that then you should also remove your heater hoses from the heater core or it will still heat your passenger compartment a little just from radiant heat. You would want to block it off at the "cowl cover" itself though so no more water gets in. You can get cheap plastic vent covers. this would be the easiest and quickest fix. Here are some pics of mine with the cover on and then off during my repair. Hope this helps.
My pics were too big for this forum so I e-mailed you a couple.