Why run hdmi instead of toslink

Full disclosure. I am not starting a cable war, but I’m really curious why video is routed thru any audio equipment at all. I don’t need atmos or crazy audio. I have a standard 5.1 setup with Khorns and a LaScala for a center speaker routed thru a classe 800 ssp and krell 3250(2). I use toslink out from the monitor in. 
Sound quality is fantastic, so looking for valid reasons to upgrade processor to support the 4K. With 8K out now and I guess 16k coming at some point, does it make sense and to keep upgrading?

thank you in advance. 
Audio in movies is a bit different from songs on CDs or high-res / DSD files.

The bit rate in kbps is sometimes variable, not CBR; other times it's comparable or slightly better than CD quality which is: 16 bit by 44.1 KHz & 1411 kbps.

If you want to upgrade later it would be for better visuals. With say, an 8K TV, you would need an HDMI cable that is suitable for that resolution.

Austere makes some nice ones.