Why did the level of the floodwaters need to carry on increasing until they were fifteen cubits above the level of the mountains?
(7,20) “Fifteen cubits above did the waters strengthen, and the mountains were covered.”
Rashi explains that the posuk means that after the waters reached the top of the mountains they continued to increase a further fifteen cubits. But what was the purpose of this increase? Once the waters reached the level of the mountains, Hashem’s goal to destroy everything on earth was achieved, so what need was there for a further fifteen cubits of water?
It seems to me that the answer to this question will also explain a perplexing commentary of Rashi on posuk 8:4, where he makes a calculation which shows that the ark was submerged eleven cubits in the water. But why does Rashi need to teach us this - Rashi purpose in his commentary on the Torah is to explain the plain understanding of the posukim, not to teach us unnecessary trivia!
But it seems from our question that it was not sufficient that Noach and the others were saved physically from the floodwaters by the protection of the ark - they also had to be removed from the area of destruction. This was achieved by further increasing the waters so that the ark rose above the level of the mountains.
But why was an increase of fifteen cubits needed for this? The answer is because we know from Chazal that the domain of a person or an object - and thus its influence - extends four cubits in each direction. Thus the ark had to rise four cubits above the mountains in order to escape the sphere of destruction, and since the ark was submerged eleven cubits in the water as Rashi crucially explains, a total increase of fifteen cubits was needed to achieve this.