If you're streaming music through your MAC or computer, you don't really need a streamer.
If you have a dedicated 2 channel system that doesn't have a computer the streamer provides the functionality you get from the computer - internet connectivity to download/stream files, an interface to select music (often through your phone or tablet), and the firmware/hardware to decode the digital signal and send it to your preamp/amp. It's basically another source, like a CD player, turntable, etc.
You may find that a dedicated streaming device provides better sound quality than your computer.
If you have a dedicated 2 channel system that doesn't have a computer the streamer provides the functionality you get from the computer - internet connectivity to download/stream files, an interface to select music (often through your phone or tablet), and the firmware/hardware to decode the digital signal and send it to your preamp/amp. It's basically another source, like a CD player, turntable, etc.
You may find that a dedicated streaming device provides better sound quality than your computer.