Base layers without a gusseted crotch use standard construction where four seams meet at a single point in the crotch area. This is the most common design you'll find, especially in budget-friendly and casual ski underwear options.
For casual resort skiers who stick to groomed runs and don't spend much time in deep bends, standard construction is usually perfectly fine. You might not notice any difference during a typical day on the slopes.
The downside is that those converging seams can create pressure points and chafing during deep bends or long days. Snowboarders are especially affected since they constantly bend at the knees and sit in the snow. If you're an aggressive skier or rider who moves a lot, the lack of a gusset could become uncomfortable by afternoon.
If you're shopping online, don't assume all premium base layers include gussets—many don't. Check the product details if freedom of movement is a priority for you.