Articulated knees are pre-bent at the factory to match the natural flexed position of your legs while riding, rather than hanging straight like a regular pair of pants. This means less fabric bunching up behind your knees when you're in a skiing or riding stance, which translates to better comfort and easier movement throughout the day.
If you're an active rider who spends the day carving turns, racing, or just bending your knees a lot (which is most of us), articulated knees are absolutely worth seeking out. The reduction in fabric restriction is noticeable, especially after a full day on the mountain. You'll feel less fatigue and more freedom to move naturally.
For casual riders who mostly cruise on gentle groomers or just want pants for occasional trips, non-articulated pants will still work fine. But since the comfort upgrade is significant and often doesn't add much cost, articulated knees are one of those features that's hard to go without once you've tried them.