Sticking a needle in some part of your body isn't usually the cheapest, easiest, or least unpleasant way to deliver treatment. But sometimes it's the best way, and I can think of 3 reasons why:
#1 The treatment can't be delivered any other way - Some medicines don't come in any other form than injections, like insulin, certain antibiotics, or vaccines.
#2 The patient won't tolerate it any other way. - If a patient has vomiting and diarrhea that just won't quit, you can waste as many anti-nausea pills or suppositories as you want trying to make it better. You can't throw up an injection.
#3 The treatment will work faster/better as an injection. - Back to the vomiting/diarrhea scenario, if someone is dehydrated enough, replacing fluids and electrolytes directly into the blood circulation can tremendously speed recovery.

There are 4 basic places the needle can go: just under the skin, into the muscle, into a joint, or into a vein. (to be continued).