Depth buffers only work well for opaque objects, which cover each other completely. With partial coverage or blending, multiple objects could end up contributing to a pixel in an order-dependent way (e.g., if you're viewing an anti-aliased leaf edge under the surface of water through a window). The depth buffer, which only stores a single depth, doesn't solve this problem directly -- you can use it to render each transparent layer one-by-one ("depth peeling"), but this can be slow.