Firing smoke into a hex with an enemy unit gives an extra +1 for firing out of smoke. So they're making it that little bit harder for you to get that effect by firing smoke and a specific unit
that's just a thought
firing at anything concealed is harder.
There are 10 to 12 guys standing in a 40 m area that you try to put smoke on
whereas firing in a big empty hex 40 m x 40 m the fire and I suppose
Makes the same sense for me dropping a mortar round on top of them, it's easier to hit them in a 40 m area, it's a hell of a lot harder to hurt em
But this is a realism argument
the simple answer is because the rules say that it's +2 more to hit something that is concealed
Oh, then here is a bonus point to. In our examples above there is only one unit of each side on the board per se.
Laying smoke in an empty hex in front of a given unit often provides more cover because more than one specific unit may have its line of sight affected
just a tactical consideration of course