A PC is not designed to just play music but a streamer is. You can get a lot of electrical noise from a PC which can be avoided with a streamer. A streamer like the Node 2i can receive it's signal wirelessly and placement is not restricted or it can be hard wired straight to a router. It's invaluable for me to be able to heard the Ultra hi res content from Amazon Music HD at it's best.
I think the part I bolded above is the main draw of a streamer over a PC for most people. With a wireless streamer, you can play any song available on your streaming services, and with your smartphone or tablet, have total control from your listening chair without having to stand up and walk over to the equipment rack. With a PC, you have to either place the PC next to your seat (and run at least 1 wire across the middle of the floor in your listening room) or get up any time you want to change your album/playlist/song etc.
Or if you're listening just for background music while doing other things, it's again really nice to be able to control the music from anywhere in the house with your smartphone. Not possible with a PC.
I guess we (myself included) are willing to pay a lot for convenience (or the ability to be lazy? haha)