Most of the time, when you talk to someone about striking, they will assume you mean using a closed fist. It’s a fairly natural human instinct to hit with a closed fist. In addition, closed fist fights are what we see in most combat sports. (Though Pancrase was an exception to this.)
So we should always hit with a closed fist, yes? No. In terms of training for self defense, there are plenty of uses for open handed strikes. Think eye pokes, groin slaps and palm heel strikes as an opening salvo of open handed shots. The brow ridge and cheek of the human face are constructed in such a way that it’s tricky to fit a fist into someone’s eye (should you so desire to do so) and so extending your fingers makes the task much easier. Striking to the groin with an open palm adds a few inches of reach and thus allows for more room for error. Palm heel strikes to the head are often safer (for the striker) than closed fist strikes as skulls are hard things that can easily break one or more of those little bones in the hand.
Close your fist, open your fist. Experiment. There is room for more than just a simple punch.