[Imp]

Generally keeping units spaced is advisable mainly because of suppression, you are letting incoming fire affect multiple units rather than just the target.

As mentioned many weapons can suppress the adjacent hex as well, all can suppress other units in a hex. Fire a MG at a group of adjacent armoured vehicles to hear the ricochets or turn blast circles on to see it.

There is a small chance it may actually damage any unit in the target hex also.
Put a group of trucks in same or adjacent hexes & fire HE at them to see the effect, the target might survive while other units get toasted.