I've read in the news about the rule regarding fuel left
Vettel was disqualified from second place after his car was found to have less than the mandatory one litre of fuel required for post-race testing. [...]
stewards said that the article in the F1 rules relevant to the case "unequivocally call for a remaining amount of one litre [of fuel] and does not allow any exceptions under which circumstances or for what reasons" less was available.
Therefore, the reason for the lack of fuel [a leak] was deemed "not relevant" to the disqualification, and the team's case was rejected.
What's the official motivation for requiring at least one liter of fuel left in the tank?