That's an RFID pass pocket, designed for resorts that use RFID scanning at lift gates. Instead of fumbling to pull out your pass or wearing it on a lanyard, you just leave it in that pocket—the scanner reads right through the clear window as you walk through the gate.
The pocket sits on your lower left leg because that's the side most resort scanners are positioned to read from. It's a small detail, but if you ski or ride at an RFID resort, it's genuinely convenient—you glide through the gates without thinking about it.
If your home mountain uses RFID or you travel to resorts that do, this feature is worth having. If you mostly ride at places with visual pass checks or venture into the backcountry, you won't miss it. You can always use a lanyard instead, but the built-in pocket is cleaner and more secure.