Drilling accurate, consistent holes can be tricky, especially on thicker posts. Not to mention the difficulty of holding a drill level and steady. The drill guide provides stability while drilling, so your holes turn out perfectly every time.
If your project includes stairs, the single-hole template is especially helpful for angled cable pass-through holes. The adjustable head pivots and locks in place, so you can accurately drill parallel to the angle of your top rail.
The drill guide also helps keep your holes even and on center all the way through your post, so you can avoid the look of uneven cables.