Anti-microbial pads are treated with silver-ion or similar technology to slow bacteria growth, which is what causes that funky smell after repeated rides. If you ride several days a week or sweat heavily, this feature genuinely helps keep your helmet fresher between washes.
This treatment is most useful for commuters and frequent riders who may not have time to wash pads after every ride. It's commonly found in mid-range to premium helmets, so you're not usually paying extra just for this feature alone.
One thing to keep in mind: the anti-microbial treatment does wear down over time, especially with regular washing. So while it helps extend the time between washes, you'll still want to clean your pads regularly and replace them when they lose effectiveness.