Music Server With Hard Disk Question.

I have never understood the reason behind an audio device that has an integrated hard disk of a huge size and still has an ability to stream the music from a computer. Why would someone want, say, 400GB full of music and still stream from a remote device? Can it just be a feature that allows one to copy music from that remote device and that's it. If the question is limited space, most of such components are equipped with USB ports and are capable of recognizing external hard disks so you can add another terabyte to be happy.
What you think?