The Empire State building was built at a time when great, big airships called zeppelins were still in use. For this reason, the Empire State building was constructed with these zeppelins in mind, and the point was meant to be a docking zone for a zeppelin!
Although the spike was an attempt to be practical, now—with zeppelins long out of style—it is simply interesting architecture. Unfortunately, the zeppelin landing was only ever used once. Because the Empire State Building was so tall, winds were high at the top of the tower, and docking there was too dangerous to be practical.
The other problem is that it was also pretty close to the end of the age of airships. The Empire State building is well-known architecturally, not only for its point, but also for its shape. Rather than be a large rectangular prism, like the soon-to-be popular International Style, the Empire State building was made with a series of indentations.
Because of these indentations it has a distinctive appearance, and was actually designed especially well considering its height. It was so well-designed that a B-25 bomber that crashed into the building did not severely damage the structure.