I use filler to 1) reduces reverb, 2) add hefty feel, and 3) counter-balance extra heavy spring I use just on the spacebar.
Not sure how it is with others but I need the extra weight to offset my tendency to press spacebar prematurely. Cherry black spring is heavy enough to reduce typing error but it sometimes feel too heavy in-use so I end up adding or removing fillers to fine-tune until the perfect balance is reached. I wasn’t able to use filler this way until I found Blu-Tack but now that I can, it’s been great.
The filler just removes reverb by removing the room for echoing and does nothing against other types of noises. Extra weight striking the PCB may even increase the loudness of noise if switch spring is too light.