Assault seems similar to "attempted battery", so it feels akin to charging "attempted murder" whenever "murder" charges are brought.
Edit: because this question has tagged the United States, the answer below discusses US law, not the law of any other country.
Because they're separate crimes that, as a general rule, don't merge, and prosecutors like to charge multiple crimes when they can because it gives them leverage in plea negotiations. You could also charge attempted murder if there's been an actual murder, it's just that it would kind of be a waste of time since they would merge upon conviction.
You're close on the view that assault is attempted battery. But, note that generally assault is placing someone in imminent fear of receiving a battery, whereas as an attempt usually requires a 'substantial step' towards commission of the actual offense, so the ambit of assault is slightly larger than the ambit of attempted battery (again, generally speaking).
Assault may be defined as:
threatening or attempting to inflict immediate offensive physical contact or bodily harm that one has the present ability to inflict and that puts the victim in fear of such harm or contact
Whereas Battery is the:
intentionally or recklessly causing offensive physical contact or bodily harm (as by striking or by administering a poison or drug) that is not consented to by the victim
Battery is almost always preceded by an assault, which is why the terms are often used transitionally or combined, as in "assault and battery."