How can they not effect the sound

This sounds a bit weird but I’ve seen it done easily & successfully for a dented in soft dome Morel tweeter: simply use your mouth  barely touching the tweeter & suck in like your drinking from a straw. Instant fix & sounded identical once again to the other speaker. 

I’m sure there’s a teeny tiny difference if there’s a dent on any driver, but I doubt very seriously that most anyone can hear the difference.

I had a dealer who built his own speakers back in the day and he would often remove the dust cap and get better performance..  He said it wouldn't work with all drivers.

Small dents are inconsequential but large ones can certainly degrade the wave launch. Regardless, it’s surprising how many audiophiles manage to put large dents in their speaker cones/domes. Like, respect your gear. 

On metal cone drivers, such as on KEF speakers, small indentations will not impact the sound quality of the driver (per KEF). It’s more the mental anguish of putting a blemish on a beautiful speaker as well as resale value.