binau said: "is there any reason to do this at NYTW beforehand?"
IF in fact a Broadway transfer is already confirmed behind the scenes (which I'll believe when it's announced), my guess is that the purpose of the NYTW run is to build buzz, and a public impression of demand.
Merrily isn't a well-known property outside of a very niche group of cult Sondheim fans. And even within the theatre community, it has a spotty reputation. Radcliffe is still a big name, but it's not 2011 anymore, and he might not be able to recoup a Broadway musical that has limited mass appeal.
So they put it in a 200-seat "prestige" house, where it will be an extremely hot ticket. As people clamber for tickets, the buzz will build, and the production will pick up steam as a desirable commodity for both investors, and for a certain segment of the public.
Whether that segment of the public will be large enough remains to be seen, but regardless, that's the theoretical strategy at play.