I have a ROG STRIX B450 build running like a champ, my GPU takes up the top slot, but I have a x8 slot under it that I feel I should be using for something to speed up or add functionality to my PC.
What do most people use these for or do they leave them empty?
For starters, taking a wild guess, I would say most people leave the slot empty as many motherboards split 16 lanes of PCIe between the top two slots. Adding an AIC to that slot could push your GPU down to x8, and while we can go back and forth on whether or not it will affect performance, many don't bother.
That said, your empty slot does have a wide range of options for adding storage or connectivity to your machine. You could go AIC NVMe and put in a super quick game drive or add premium sound to your machine with a sound card.
Some even use those slots to upgrade the networking capabilities of their PC with Multi-Gig adapters or even Wi-Fi.