I think they divided some shots between L.A. and NYC for several reasons. Some exterior establishing shots (e.g., the Daily Planet building, etc.) used L.A. area settings for convenience (e.g., L.A. City Hall, Griffith Observatory, etc.), since the show was shot in the L.A. area. But some of the stock footage, such as when Superman is flying past the urban buildings, or the other urban shots (e.g., showing the Manhattan Bridge) are used because they want to give the impression of a large, gritty, iconic urban landscape to go along w/a lot of the episode plots. And there's no more iconic city than NYC.

They also make many references in episode dialog that hint strongly at NYC as the likely model for Metropolis: the Chicago White Sox are coming to play the local team for an evening game; someone is going "up to Boston"; and in the episode "A shot in the Dark," there are shots of the NYC subway system, including the AA local to 168th Street in Washington Heights (NW of Yankee Stadium), and the iconic 34th Street subway station.

For more info, here is a link to a discussion of the aforementioned "A Shot in the Dark" episode. Note that there was actually a subway line in Los Angeles at the time the episode was filmed, but I think the NYC line is more iconic and a better choice for stock footage.

And here is another link. I'm just pasting it because the link tool here doesn't work correctly when a web page addy ends in a right parenthesis:

This is location shooting information for AoS:

https://en.wikipedia.org/wiki/Adventures_of_Superman_(TV_series)#Locations

Thanks for the explanation and links.