A dial or ratchet retention system uses a small rotary knob at the rear of the helmet that lets you fine-tune the fit with precise micro-adjustments. The biggest advantage is one-handed operation—you can tighten or loosen the helmet while riding, which is handy if it feels loose on a rough descent or too snug as your head warms up.
This system is the most common on mid-range to premium helmets for good reason: it offers the best balance of adjustability and ease of use. It does add about 15-30 grams of weight compared to simpler systems, but most riders find the convenience well worth it.
If you ride road, gravel, or commute regularly and want a helmet that stays put without constant fiddling, a dial system is a great choice. Just be sure to try it on before buying—some dials are easier to reach and turn than others, and a system you can't comfortably adjust defeats the whole purpose.