Once upon a time, the Star Ferry was the only way to travel between Hong Kong Island and Kowloon, then the car tunnels came, and then the MTR. If you're taking the Star Ferry, you're doing it for nostalgia or because you're a tourist. It's a great way to see the skyline of both Kowloon and Hong Kong Island.
The bonus is that the Kowloon ferry terminal is at Tsim Sha Tsui and the Hong Kong Island terminal is at Central near the IFC, the world's largest shopping mall. So tons to do very close by the terminals. Als each ferry terminal is close to an MTR stop.