Actually, it's the "help" part that's doing that - "@target" just means that it's checking your target, which is default behaviour.
In fact, this will work just as well, and use fewer characters.
Code:
#showtooltip
/cast [help] heal; smite
#showtooltip by itself shows the tooltip (and icon, if you used the ? icon) of whatever spell is going to be cast, based on conditionals and modifiers. In this case, target a friendly and it'll show "Heal", otherwise it'll show "Smite".
Also, as Seerah pointed out, the /stopcasting line and the one following it were completely redundant. This macro will do the same thing, but is quicker to type, easier to read, and uses fewer characters (not important for this macro, but can be important for more complex ones).