The reason being you can train the same movement in a resisted situation. Without having to kick people in the groin. a perfectly reasonable way to gain timing and power with a technique is to train it against a resisted opponent. This is difficult with a groin kick as very few people will get kicked in the groin on purpose. So to gain the timing and power needed we can use an existing kick and change the angle a bit. this kick is effective to the inner leg anyway. The set ups are the same. The openings are the same. If you can reach the inner leg. You can reach the groin. kicking to the leg will provide a real reaction that you can work the rest of your combinations off. i prefer this to relying solely on pads or compliant drills.