What is a liner for?

What is a liner for depends on type, but generally liner is inner layer or insert used to provide protection, warmth, comfort, waterproofing, or structure to outer item. In clothing, liner is for adding warmth, such as quilted liner inside winter coat traps heat, making coat more comfortable, hiding seams, and making garment easier to put on. In bags, liner is for protecting interior, organizing contents, and adding durability. In makeup, eyeliner is for defining eyes and enhancing appearance, lip liner for defining lips and preventing lipstick feathering. In construction, pond liner is for holding water and preventing leaks, trash bin liner is for keeping bin clean and containing waste. In swimming pools, pool liner is for waterproof barrier. In general, liner is for providing extra layer that improves function, such as insulation, moisture barrier, protection from wear, comfort, or aesthetic enhancement, acting as barrier or inner layer between outer shell and contents or body.