A streamer is just a device that allows you to play music that is stored elsewhere. The music could be stored on a NAS, or on an ordinary computer like Abe says, or cloud storage, or an internet radio station, or service like Tidal.
By some definitions a streamer includes a music server with local content. You'll see the term streamer used in both cases but as the industry evolves we are starting to see a shift more toward your definition.
If you only ever listen to music on your main system, and are happy with your computer audio setup, then a streamer doesn't make sense for you. But if you are like most people and listen to music on multiple systems/devices, you might consider streamers as way to make your music collection available in multiple places without have to maintain multiple copies of your music library.
I have one copy of my library on my local disk in my main system (Mac Mini) and can also stream content from that single libary to multiple devices w/o the need for more than that single library.
Apparently Aurender sees a market for streamers with local storage. Otherwise they would not be making the X100 series with upwards of 12TB internal HDD.
