From what I could gather from the posted comments at the worldsfaircommunity.org forum (hat tip), the arch was one of eleven scattered about the grounds of the Worlds Fair and in essence served as an advertisement for the General Foods Company as well as an information kiosk and rendezvous destination for fair goers. After the fair, General Foods sold off and/or scrapped the kiosks and the good old Worlds Fair enthusiasts at the linked website above were able to track down five of them that still exist in various parts of the country.
One of the archs ended up at the entrance to the Cherry Valley Center, a.k.a. the Pathmark shopping ctr, and remains WH's best specimen of the type of googie architecture that was popularized in the '50 and '60s.
I like to think of it as West Hempstead's own Gateway Arch, beckoning homesteaders' westward migration into the wild wild west (Elmont?) and the hinterland beyond (NYC?) - OK, maybe the comparison's going too far.
I'd love to find out more about this landmark if anyone has more info about it (e.g. when it was put in, who bought it, etc.)