A height-adjustable chest strap lets you slide the sternum strap up or down along the shoulder straps, so you can position it right where it's most comfortable and secure. This matters more than you'd think, especially if you ski in varying conditions throughout the season.
The biggest advantage shows up when you switch between thin base layers on warm spring days and bulky insulated jackets in mid-winter. A fixed chest strap might sit too high or too low depending on your layers, causing discomfort or letting your shoulder straps slip. With height adjustability, you just reposition the strap each time your layering changes.
It's also helpful if you're between sizes or share a pack with someone who has a different torso length. While it's not a must-have feature, height adjustability is one of those small details that makes a pack noticeably more comfortable over a full day on the mountain.