There are a couple of cases for offshore wind in the US:

- To supply power to densely populated coastal urban areas, where there isn't a lot of free land for onshore farms nearby.

- To provide some after-dark generation in areas where daytime solar power already surpasses the limits of demand.

The second case gets weaker if there is room for onshore wind power in the same region, or if storage prices keep falling so that solar electricity can be used after dark. The second case gets a little stronger if a region's highest seasonal electricity demand occurs in winter instead of summer.

To add some more points the others have made:

- You can make the turbines substantially bigger offshore than onshore - There's better wind offshore - Both the previous ensure there is a higher capacity factor - There's a lot more flexibility in connecting to the grid. Grid accessibility is a huge challenge in siting onshore projects. You either need to be near an access point or conquer the immense, years long hurdles of constructing new transmission lines - The farms can have a large number of turbines which makes capital and labor more efficient

It's really an economic equation that you have to optimize for our moronic system of law. Do you want to go to war against Sea NIMBYs or Land NIMBYs? How much energy can you generate with a windmill weighed against how much it will cost to litigate against people who claim you are giving whales cancer. How much energy can you generate with solar panels and how long will it take to prevail over the people who say you are wrecking the habitat of the critically endangered San Benito Burrowing Amoeba.

Because offshore wind can be even cheaper than onshore solar, basically. It all depends on local grid conditions of course.

In this example, Long Island being a giant island is still out of land remaining to build any large scale power plant, and the island along with NY is in dire need of more generation capacity.

Because NY state isn't going to be the most efficient place to put solar farms. And running distribution long distances is also expensive and inefficient.

And shut down the country at 4PM in the winter.