... It is also convenient to have an "all in one" internal package, rather than an external box powered by a wall wart (or worse yet, batteries).
It is definitely a trick sometimes connecting my tube amp to a wall outlet to use headphones with my computer. I bought a Monster power strip (bundled free with my TV), but it introduces grounding noise in the background that I guess is due to the transformer or power-conditioner built into it, so all the computer bits (& my Xbox) are plugged into it and the tube amp's wire leading to it's wall-wart stretches across to the other power outlet on the other wall of my room. I usually use my FiiO Mont Blanc with my computer/Xbox instead, the battery power is a boon for fewer wires and usually comes with me to charge on my nightstand table next to my bed every night after I'm done with music anyway. I could see someone being frustrated if they often ran out of battery power, though.