Nurse's need to be familiar with the muscular system for several reasons. First, IM(intramuscular) injections are a very common way to deliver medications. The age of the patient will dictate what muscle to use for this purpose. Also, the specific medication dictates which muscle to use. There are a few common muscle sites used to give IM injections such as the deltoid(shoulder area), gluteal's(buttocks), vastus lateralis(lateral thigh, for kids).
If a nurse gives an injection in the wrong site, this can harm the patient. Harming the patient is the absolute last thing you want to do. You are there to help, not hurt.