Internal helmet storage means your helmet fits inside the main compartment of your pack rather than clipping to the outside. The main advantage is protection—your helmet stays safe from scratches, dings, and getting snagged on tree branches or chairlifts. It's also secure, so there's no chance of it bouncing off during a skin up.
The downside is that a ski helmet eats up a surprising amount of pack volume—often 5-8 liters depending on helmet size. In a 20L pack, that's a huge chunk of your storage. It's also not quick to access; you'll need to unzip and dig around whenever you want to put your helmet back on.
Internal storage makes the most sense for hut trips or long backcountry tours where you'll be stowing your helmet for extended periods and want it protected from gear piles and rough handling. For most resort days where you keep your helmet on from first chair to last run, it's not a feature worth prioritizing.