Assaults are dangerous
A detachment of troops must be kept behind the main line at all times. If, at any point, a unit breaks then you can use these reserves to fill the gap.
Assaults are dangerous, even with all the modifiers in your favour there is still a 1 in 36 chance of only getting a Forced Back result.
Firefights are useful for moving an enemy backwards, not for wiping him out.
Assaults are dangerous, dedicated detachments should be constructed to perform assault work.
Space Marines without Transports are expensive defensive troops.
Assaults are dangerous but they really allow you to pile blast markers onto the enemy.
It doesn't matter if a detachment has 1 or 100 units in it, it will still wipe out a broken enemy.
Assaults are dangerous but are usually the final deciseve move that can win a battle.
By the way, did I metion that assaults are dangerous?