A laptop is a keyboard-oriented device, packing a,lot of computing power. It has the shortest battery life, and requires a desk, or at least a seat, to operate comfortably.
A phone is a handheld device, one which is always on you. It necessarily has a smaller screen, no separate keyboard, and only has a decent battery life because it sleeps most of the time. It can have serious peak performance, but has a hard time sustaining it (ask Ingress players).
An iPad (or another tablet if you feel fancy) is a two-hands device with a good, easy to use touchscreen, and software built around it. It's a natural choice for a graphic artist, for instance. An iPad has a more tolerable onscreen keyboard than a phone. Also its screen is large enough to enjoy movies, etc, so it's a convenient visual media consumption device, more so than a phone. I had laptops with touchscreens; they were fun, but not nearly as handy.
(For the record: I travel with a phone and a Linux laptop.)