“23rd Street” always sounds like a Manhattan stop to me. But I think they still call it that, they just replaced the Ely Ave part with Court Square. Some people find it easier to think of places in terms of landmarks so “Court Square” has more meaning to them then “Ely Ave”. “One Court Square” is the Citigroup building, probably the main function of that particular stop is that building so they felt it makes more sense to name the stop after the building rather than some random cross street. There are many stops that are named after landmarks such as World Trade Center, Penn Station, Port Authority, Bryant Park. For people who aren’t regular subway users this can be very helpful if they are looking for one of these, they don’t have to remember the cross streets. This is just the city trying to be more user-friendly. I guess it does cost a bit to change the signs, recordings, etc, but in the long run it makes the subway better.