Midweight ski socks hit the sweet spot between thin performance socks and thick winter socks. They have enough cushioning to keep your feet comfortable all day without adding so much bulk that you lose the feel of your ski boot. That's why they're the most popular choice for everyday resort skiing.
If you're skiing in variable conditions—cold mornings that warm up by afternoon, or trips where the weather shifts—midweight socks give you solid warmth without the risk of overheating. They work well with most boot fits, so if your boots feel pretty good already, midweight is a safe bet.
The one tradeoff is slightly less boot feel compared to lightweight socks, so racers and aggressive skiers who want maximum precision might prefer going thinner. But for most recreational skiers, midweight is the versatile do-it-all option that keeps feet happy from first chair to last run.