Whether or not a unit has moved can make a big difference to its firing. If the warriors hold a position, take up firing stances and aim at their targets properly they can hit targets further away than if they are firing on the move. In fact some weapons are so heavy they can only be used if their firer halts to brace themselves. For the time being it's enough to know that moving makes a difference to a unit's shooting. The most important thing to understand is that the whole unit counts as moving even if only some of it moved in the movement phase.
Moving and Shooting
Source: Core Rulebook 3E, p. 47
Whether a unit moved affects its shooting; the whole unit counts as having moved even if only some models did.