Close Order - in the Ranks
For confronting the enemy, this is the preferred spacing in the ranks. The soldiers are concentrated into a unified front to face the attack - or to make the attack. No longer are there individual soldiers but a single force. The close spacing between each does not permit any enemy to penetrate the unit without a hard fight.
Close order may also be useful for narrowing ranks to fit a column on a thin stretch of road during the march or through a crowded area, such as camp, a throng of thrilled spectators or the entrance of a beer tent.
On the definition of what constitutes 'Close Order,' if you wish a little diversion as well as fodder for discussion with friends (or argument with not-really friends), then detour here.