I have no idea why they do it exactly that way. If it's on an outside wall, you cannot drill a full-sized hole through the plate because of the rim joist beneath it.
Doing it this way uses more fittings than necessary, it seems to me. The method i was taught is to first drill a hole in the plate, then use a sawzall to cut the face of the plate out. That leaves enough room to drop a piece of pipe with a 45 on it into the floor, and the hole in the plate is covered with a steel plate.
By using my method, there two forty-fives necessary to go horizontally toward a main or to go around the edge of the concrete foundation. By the method shown in your picture, you've already got two forty-fives, you'll need yet another 90 degrees of turn to go horizontal, and from what I guess, on an outside wall you still aren't clearing the concrete foundation so you'd need yet another vertical offset to go around that.