Why, to get access to the UN Library, of course!!!! ..... NOT!
There was a post here a while back that suggested that it was intended so that when the cry of "peace & security" was announced, JWs would be ready, because if they are on the inside of the UN, they would hear it before the general public. Problem with that is twofold: (1) The "peace & security" cry was to be heard clearly throughout the world; there is no need to have "insiders" to hear about it; (2) unlikely that being an NGO would actually make a group privvy to inside information; being an NGO is not the same as sitting on the security council.
Maybe this was "spritual warfare"; in war you always want to have your spys planted behind enemy lines; so manybe they wanted to be like James Bond!
~Quotes, of the "Sean Connery" class