Sole reinforcement adds extra-durable knitting to the bottom of the foot, where abrasion from your boot's footbed can wear through socks surprisingly fast. It's not essential for everyone, but it makes a real difference if you're logging lots of days on the mountain.
This feature is especially worth considering if you're a high-mileage skier out several days a week, or if you do a lot of walking in your boots—like hiking to lift lines or navigating base lodges. It's also handy during the boot-fitting process, when you're repeatedly sliding your foot in and out as adjustments are made.
Beyond durability, sole reinforcement adds a touch of cushioning underfoot, which can take the edge off hard landings or long groomer runs. If you've ever worn through the bottom of a ski sock mid-season, this is the feature that prevents that.