For me, spray bars tend to distribute the water move evenly. This is especially important when your whole tank is crowded with plants. Using conventional method of filtering, your output is only from a single source, which will be usually blocked by your jungle of plants. Therefore circulation is only apparent in a confined area around the output of your filter.

With the spraybar, you've a spraying galore. Your plants get love equally, everywhere. That's my theory of putting my own spray bar at least :P