Adding the airship routes to Ishgard is mostly just for story purposes. If you remember, at the start of Heavensward, Ishgard was sealed off from the rest of the world. No one from the other stretches of Eorzea was allowed into or out of Ishgard territories.

The airship routes being opened doesnt just affect the players, but the world. The people living in Ishgard, and the people living in the other various scattered regions of the world can now use the airships to travel. In other words, thanks largely to all the duty we have done for Ishgard, we have caused them to lower their barriers, open the borders, and allow people in.

While the airships may be useless to a great many of US, adding airship routes was more symbolic than practical. It's just a bit of world building. This IS a role playing game, afterall :P