1.) It gives you housing security, rents are not dependable from one year to the next.
2.) A co-op is responsible for the maintenance of the whole building, unlike a condo.
3.) As a buyer you can inspect the financial records & maintenance records of the building before you buy.
4.) If you buy into the right building it will at least appreciate at the same rate as inflation.
5.) If you buy into the right building & make the right improvements while you are there, you should be able to sell it for more and sell it quickly with the help of the right Broker when you are ready to sell.
6.) The closing costs to purchase a co-op are less than the closing costs to purchase a condo by several thousands of dollars.
"Disadvantages". Some may say that the co-op board application process is a disadvantage, because the board has to approve who buys in. This is actually an advantage, there are many Condo's that are now un-salable due to too many investors buying or owners that do not want to make needed capital improvements etc.
The key to buying a Co-Op is buying into the right building in the first place, they all run slightly differently. Think of it as buying stock on Wall St, each company runs slightly differently and the stock will trade accordingly.
When you are buying a Co-Op you are buying stock in the company that owns the building, as a share holder you have the responsibility of looking after the whole company that owns the building.
When you buy a Condo you have no such control of the whole building.
For a place to live in NYC a Co-Op is the best most stable option between; renting, or buying a co-op or a condo.