And a metaphor isn't the thing it is used to convey. So?

Are you saying you where talking metaphorically? Because the metaphor compares two similar things.

Also, why is a metaphor needed here?

>Are you saying you where talking metaphorically? Because the metaphor compares two similar things.

A metaphor compares two things similar from a certain aspect -- they could still differ in every other way. One could use a metaphor from agriculture to clarify their point on a political issue, for example.

>Also, why is a metaphor needed here?

For the same reasons that it's needed elsewhere. To get the point across in a simplified way, because people can understand a phenomenon better in one domain than they can in another (aka "domain dependence").

> A metaphor compares two things similar from a certain aspect

When I said "the metaphor compares two similar thing" I didn't mean metaphors in general, I meant your metaphor specifically compares two things that are similar. The relevant differences between NY and the moon do not exist between NY and Illinois.

> For the same reasons that it's needed elsewhere

Metaphors are often of dubious value. They can be used to clarify unfamiliar situation, where the bounds of the metaphor are clear; or, perhaps as in this case, encourage similar thinking about two related concepts.

But you provided a metaphor here without providing clarity on what the point was. When I think "why would I build a city in Illinois, when there already one in NY", none of the answers I come up with apply equally to the moon. e.g. land is cheaper/convenient (not on the moon), the scenery is better (maybe true, but not to outweight other more pressing concerns) etc.