Attempted assault is a bit weird, simply because the crime of assault is typically defined to include both attempts at and successes at causing bodily harm to another.
That said, the concept of punishing attempted crimes is well established. After all, if one of the goals of having a criminal legal system is to prevent crime, we should punish those who attempt to commit crimes.
If someone fires a loaded gun at you, and you're lucky enough for them to miss, they should still be punished for the attempt to inflict harm on you or potentially kill you.
We can certainly debate whether the legal system should take more of a "no harm, no foul" kind of approach, but I hope the explanation above helps to clarify why our current legal system penalizes attempted and successful crimes.
