Mostly cosmetic, but it's better to have them and not need them than to need them and not have them. My most recent acquisition came without and I ordered three ... only the geekiest of geeks would know (if they got close enough), but it makes me feel better.

In general, I try really hard not to use them, but sometimes a certain room might bring out a funky overtone in a certain drum and I'd rather not futz with a drum that i know is perfectly tuned.

Al

As stated by some already- I hardly use them but when in a 'quiet' setting they really help with volume (deadening and losing resonance BUT...) making the drum thud instead of sing.... rarely used... (They do help if you trigger by reducing sympathetic vibration causing false triggering)