Photoshop will also add noise when you convert from 16 bit to 8 bit. The reason for doing this is to prevent visible banding when blurring things, or just working with gradients (like blue skies). This is really important when printing things, because sometimes banding that wasn’t visible on your screen will show up clearly in your prints. Also really important if you scale the colors of a gradient or blurred region later… small steps between colors become larger and there’s no way to get back to smooth.
I only became sensitive to whether software does blur math in higher precision, and whether it dithers, after working with prints and wasting $100 a pop on a large format print that came out with ugly unexpected banding. Last time I checked, Image Magick’s blur and 16->8 conversion was terrible, and has for a long time been one reason stuck in my head why I avoid Image Magick for serious work.
I don’t know how effective adding noise in 8 bits is. It’s better to work in a higher bit-rate colorspace, and only dither at the last second when converting the final to 8 bits. Well, it’s better to never use 8 bits if you can, but that’s not always possible. Hopefully Figma’s noise is at least added during the higher-precision blur operation before storing the results in 8-bit colors. Figma’s noise bias might be a simple mistake, and it might also be the reason they chose to clamp the blacks, if a developer was seeing -1s (which depending on the implementation, might underflow and suddenly show up as a saturated bright color).