It's not saying that regular insulin is only to be given IV. It's saying that regular insulin is the only insulin that you can give IV...which means you don't give other types (like levemir, lantus or 70/30 or aspart insulin) via IV. So you have many different types of insulin that you can give SQ, but regular insulin is the only one if you had to go the IV route.
So when do you give IV insulin ? Pts who have really high blood sugars (hyperglycemia) often need IV insulin to steadily bring it down. You can get a more predictable drop and you can monitor it more closely. So, if a pt has a blood sugar of 900, you would put them on an insulin drip and check their glucose level every hour. The insulin is rapidly absorbed and it is continuous. Then, you adjust the drip based on their blood sugar. If you are to give SQ insulin, there is a period where it gets absorbed...some are within 15-30 minutes while others are longer. If you are trying to rapidly and steadily bring down there blood sugar, SQ insulin isn't going to give you the results quick enough. Plus, with SQ insulin, they get what they get... but with a drip, you can increase it or decrease it every hour, so it's more effective. Once their blood suger is under control, then you can go the SQ route to maintain. IV insulin is also given to post-op cardiac pts, trauma pts, and neurosurgical pt. Well-controlled glucose levels is very important in this pt population for overall management, better healing, decreased mortality, etc.