Mesh-lined vents have a thin screen behind the zipper opening that lets air flow through while blocking snow from getting inside your jacket. If you've ever cracked open your pit zips on a deep powder day only to get a backful of snow, you'll appreciate the difference.
This feature really shines in wet, deep snow conditions—think waist-deep freshies or heavy Sierra cement. Without mesh backing, open vents become an easy entry point for loose snow, especially if you take a tumble. The mesh keeps that snow out while still letting you dump heat when you're working hard.
The trade-off is that mesh slightly reduces maximum airflow compared to wide-open, unlined vents. For most resort riders and powder hounds, that small reduction is well worth staying dry. But if you primarily do high-exertion backcountry touring where maximum venting matters most, you might prefer unlined vents instead.