Novice Question: Dedicated Streamers

Basically, I’m confused as to what function a stand-alone music streamer without onboard storage serves.  I know I’ve got some sort of glaring, mental blank spot about this.....the only thing I can figure is that there is some sort of enhanced music file processing over what one gets with most home computers and storage happens by plugging an external storage drive(s) to this sort of component??  Can someone enlighten me better about these?  The Blue Sound “Vault” has caught my eye of late but I’m wondering if I might be missing something by not looking at the “node” by the same company.  I don’t anticipate ever using any sort of music subscription service if this is important...