Seriously?
There are many, many reasons that people IV drugs.
First off, the bioavailability of any drug injected intravenously is 100%.
Say you have a drug such as Morphine where the oral bioavailability is ~25%. If you take 100mgs orally, only 25mgs is getting absorbed. If you were to shoot Morphine, you get 100mgs of the drug (assuming you have vials and a perfect IV technique). That is reason enough for many people to IV it. In this, not only are you getting a better high and a rush from the drug.. you're also saving money as you are getting 4 times the amount than you did taking it orally. The rush is a major reason people decide to IV their drugs.
Don't get me wrong, IVing drugs can be very unsafe and massively addictive.. as in the OPs case where he missed his shot. People have lost limbs from missing shots or shooting into arteries.. but this isn't a huge problem for those with a good technique that work in a sterile environment. In order to make IVing safer there are many precautions that should be taken (such as sterilization, micron filtering, using new rigs every time you IV, etc.). That is one of the reasons we have websites like Bluelight around.. to promote harm reduction. People aren't going to stop shooting because there are others out there saying 'oh, don't do it, it's bad for you' or because of programs like D.A.R.E. If you are educated on the subject, IVing can be very safe. There is a reason that hospitals use the ROA, you know.