What is this called (Dash seal???) and do I really need it?

Those look easily made yourself if need be. Or you could always wedge a pack of matches like the old days.

Damn your dating yourself sir! Back in the 80’s we would do PDI (pre delivery inspections) when I was a dealer tech on the new cars. We would find everything from matches to newspaper to cardboard. Heck I even found a tool blocking a hole then body puddy over. Back when if a sheet metal part was punched wrong they still used the part and filled in the gap with what ever the line boss had at his bench.