Snap In Window Sill
The snap in window sill (also called a pocket sill or a capture sill) is designed to drain water and moisture through the frame itself. The snap in sill "allows" water to funnel into the frame and out the weep holes at the bottom. The problem with these pocket or capture sills is that over time dirt and debris can clog these weep holes and trap water, moisture and mold inside of your window frames. This is not what the manufacturer(s) inteded, but it is often the long term result.
Why then would a window manufacturer use a snap in sill? It is cheaper to make a snap in sill than it is to cut an extruded, true welded sloped sill (see below). One example of a window that use the snap in sill is the Alside Excalibur window.