Sandwich (or sidewall) construction means the ski is built with layers stacked on top of each other and vertical sidewalls running along the edges. This is the go-to build method for performance skis because those vertical sidewalls give you significantly better edge hold on firm snow, plus greater durability and torsional rigidity—meaning the ski stays stable and doesn't twist when you're really pushing it.
You'll want sandwich construction if you ski fast, charge hardpack, or spend time in gates. It's the standard for freeride, all-mountain performance, and race skis. The trade-off is weight and cost: these skis tend to be heavier and pricier than cap-built alternatives.
For casual groomer cruising or if you're just starting out, sandwich construction isn't essential. But if you've ever felt your skis washing out on icy patches and want more confidence in your edges, this build style is worth the investment.