I get that the universe needs this ship for immersion, but of what benefit is the ship to a player?

The main benefit of the Starliner to a player is as an economic resource - it's a way (relatively safe, all things considered) to start earning credits quickly. Adjusting pricing on seats, change configurations to adapt to the most desired routes... make a business out of it. The PU team wants to make sure that players have many options with which they can make money, and the commercial captain career is just one of them. Starliners can also serve as a base for many less-structured PU goals. Want a flying headquarters, or a heavily protectred guild transport? The Starliner chassis is here for you!