Well, he's a computer, really. He has star charts and maps of every known hyperspace route in the Galaxy stored in him. He's also a mechanic, but Astromech droids like Artoo are plugged into navicomputers of starfighters and other small ships so they can save space.

Also, droids are machines with personality, and you should understand that Artoo has saved my life more than once, and I regard him as a member of my immediate family.