There are two ways to connect your computer to your audio, analogue or digital.
The sound card of the computer does the conversion to analogue.
You connect the analogue out of the computer to the analogue in of the amplifier.
Often the sound quality of the on board sound card is only moderate.
In case of a desktop, you can add a better one.
In case of a laptop you can't so a digital out is the only way to improve on it.
The output of the computer is digital. It is converted to analogue by an outboard sound card, a DAC in audiophile speak.
This requires the PC and the DAC to have a digital protocol in common.
In the audio world SPDIF is the standard to connect audio equipment digital.
This standard is less common in the computer world.
There are sound cards having SPDIF out.
You connect it to a DAC or a receiver with digital in.
In the computer world USB is the standard.
More and more audio manufactures are producing high quality audio components with a USB input.
Streaming audio players use the network to connect to a server.
It all started on the internet (internet radio) but if you can play music located on a server somewhere on the internet, you can use the same technology to play the music using a PC or a NAS as a server in your home network.
You see more and more AV equipment supporting the DLNA standard.
In principle, all equipment supporting this standard can communicate with each other.
Modern TV’s can show the pictures on your PC.
Why? Both are networked and both are DLNA compliant.