Adjustable gaiters use a snap, hook, or buckle closure to cinch the cuff tightly around your boot, giving you a more customized fit than a simple elastic band. They're especially useful if you ride in deep powder or wet snow, where a loose seal can let snow sneak in and ruin your day.
The big advantage is control—you can dial in the fit regardless of your boot size, and the closure won't stretch out over time like elastic can. If you share pants with someone or switch between different boots, adjustable gaiters adapt easily. They do add a small step to your gear-up routine, but most riders find the extra security worth it.
If you mostly stick to groomers or spring skiing, elastic gaiters will probably do the job fine. But for riders who regularly chase fresh snow or want a seal that lasts season after season, adjustable gaiters are the way to go.