Double D-ring buckles are the most secure chin strap fastening available, which is why they're standard on full-face MTB and BMX helmets. The metal rings require you to thread the strap through and back, creating a lock that physically cannot come undone from impact or vibration—no chance of an accidental release during a crash.
The tradeoff is speed and convenience. Unlike side-release buckles that pop open with a button press, D-rings take longer to fasten and unfasten, and they definitely require two hands. This can be frustrating with gloves on in cold weather, so it's worth practicing the motion at home until it becomes second nature.
For most trail and road riders, a standard side-release buckle is perfectly fine. But if you're riding aggressive downhill, dirt jumps, or any discipline where a full-face helmet is called for, the D-ring's security is a feature, not a drawback. It's one less thing to worry about when you're pushing your limits.