A removable liner lets you take out the interior padding of your helmet and wash it by hand, which is a surprisingly handy feature if you ski often. After a full day on the mountain, liners absorb sweat, body oils, and bacteria — and over time, that leads to odor and potential skin irritation. Being able to wash the liner keeps your helmet fresher and can extend its usable life.
This feature is especially worth prioritizing if you ski multiple days per week, tend to run hot, or share your helmet with family or friends. It also helps the helmet dry faster between sessions, which is a real bonus on multi-day trips.
One thing to watch: always hand wash the liner with mild soap and let it air dry. Tossing it in the washing machine can break down the foam and ruin the fit. If you only ski a few days a season, a fixed liner is perfectly fine — just give the inside a wipe-down now and then.