Multi-density foam uses layers of different firmness levels to give you cushioning where you need it and response where you want it. Softer foam sits under high-impact zones like your heel to absorb shock from landings and choppy snow, while firmer foam sits under your toe and the ball of your foot so you don't lose power transfer when you're carving or initiating turns.
The big advantage over single-density EVA foam is that you're not choosing between comfort and performance—you get both. Regular EVA is one firmness everywhere, so it's either too soft for crisp response or too firm for good shock absorption. Multi-density solves that compromise.
It's worth the upgrade if you ride aggressively, spend time in the park, or regularly deal with choppy, icy conditions that rattle your feet. It's also a smart pick if you have foot or knee fatigue issues. For casual all-mountain cruising on groomed runs, standard EVA foam will likely serve you just fine.