> I can't even see inside my CPU, why would I care if there's an empty slot?

Then, you’re not the target audience.

> Why would I pay for a piece of plastic to fill that slot that doesn't do anything?

It doesn’t do nothing. FTA: “Their sole purpose is cosmetic, though. While they light up and synchronize with your existing RGB ecosystem, they don't contribute to your computer’s memory capacity or performance.”

This is for people with transparent PC cases and memory sticks with RGB LED lighting. For example, see https://v-color.net/collections/prism-pro-rgb-memory-voclor/...:

“RGB SOFTWARE SYNCHRONIZATION SUPPORT

Dynamic RGB lighting control synchronized across main leading M/B such as RGB FUSION, MSI Mystic Light Sync, AURA Sync, POLYCHROME Sync etc. Customize lighting profiles or assign colors to each LEDs to create your own spectacular look.“

Meanwhile lots of people in the PC building community have cases with glass panels on the side, and go to a lot of effort to make the inside look a certain way. This includes things like custom sleeved cables, perfect cable management, RGB on various things.

I also have a glass panneled side to my computer, but the only RGB on it is on the graphics card waterblock, everything else is just jet black (fans, ZMT water cooling tubing, radiators etc. etc.)

I'd estimate that most consumer PC cases sold these days have a glass side panel, and have had for a while.