Moderate warmth hits the sweet spot for most resort skiers because it balances insulation with breathability. It's built for typical winter conditions—roughly 15–30°F—where you're alternating between working hard on the run and sitting still on the lift. The fabric provides enough warmth during chilly rides but breathes well enough that you won't soak through on the way down.
The problem with going warmer "just to be safe" is that overheating is just as miserable as being cold. A base layer that's too warm leaves you sweaty, and that moisture chills you fast when you stop moving. Moderate warmth lets your body regulate more naturally.
You can always add a mid-layer on colder days, but you can't easily fix an overheating base layer. If you typically run cold or regularly ski in sub-15°F temps, high warmth makes sense—but for most all-day resort riding, moderate is the most versatile and comfortable choice.