Two reasons:
1. Let's imagine that your attack strength is 250.
Now if you fight a legion with 251 remaining health, it leaves exactly 1 remaining health point! However, this legion splinter must still be attacked with many villagers in order to have a 100% chance of victory (see also: [[Combat Probability of Victory|Probability of Victory]]). This is absolutely inefficient and means a huge waste of attack resources!
2. In some cases some players want a legion not being defeated and come through to their village. (eg. when achieved resistance level 10 and want to gather the rewards for reaching the resistance levels again). Since there is still no 'flag' implemented where a village can tell other players NOT to hit legions of this village there is a common practice (just working with resistance level 10):
If you see a legion with full health (2,300) that legion should NOT be attacked.