A Law degree will help her with public speaking skills even if never gets to train in a courtroom. And if she gets skilled in international law imagine if she ended up elected president of the European Union while Dad Felipe is in Spain.
Ok, that's a stretch but it could happen. Her cousin Otto Von Habsburg, head of the Austrian Imperial House, was elected Member of the European Parliament in 1979. Regardless, Princess Leonor will be one of the most educated and well-prepared female crown royals in European history since Queen Elizabeth I of England.