An external helmet net or clip is an elastic attachment system on the front or top of a ski pack that lets you stash your helmet when you're not wearing it. It's handy if you regularly take your helmet off during hut trips, lunch breaks, or while skinning uphill in the backcountry when you might prefer a lighter hat instead.
The big advantage is convenience—your helmet stays accessible and doesn't eat up valuable space inside your pack. The downside is that it adds a bit of bulk to the outside and creates a potential snag point if you're navigating tight trees or loading a chairlift.
For most resort skiers who keep their helmet on all day, this feature isn't necessary. But if you're a backcountry tourer or someone who frequently removes their helmet on the mountain, it's a worthwhile feature to have.