Brick-and-mortar barrier
Your skin barrier is dead cells glued by fat. Wreck the mortar and water leaks out.
Grounded in the research on Stratum corneum (Wikipedia)
Know what the barrier actually is
The brick-and-mortar model describes your stratum corneum, the outermost 10 to 40 micron layer of skin. Bricks = corneocytes, dead flattened keratinocytes that ditched their nuclei and organelles, stacked 15 to 20 layers deep. Mortar = intercellular lipids. No mortar, no barrier. That is the whole game for glow, hydration, and not looking irritated.
Respect the mortar (it is three lipids in ratio)
The mortar is ceramides, cholesterol, and free fatty acids arranged into lipid lamellae sheets between the bricks. The ratio matters more than the amount. Strip them with over-cleansing, harsh surfactants, or too much acid/retinoid and the lamellae break down, water escapes, and irritants/allergens walk in. Look for products that replenish all three, not just one.