High arches are important in ballet because they allow the dancer to rise high up in releve and get over their boxes in pointe shoes. The banana-footed aesthetic is also very "in" at the moment.
A friend from ballet class gave me this quick tip on how to improve your arches: Wear your pointe shoes until you absolutely can't! Don't rely on the support of brand new shoes to help you rise en pointe, by wearing your pointe shoes down more than you usually would, you build up your arch and the strength in your feet, as well as your skill en pointe. Soft shoes mean you have to work harder, which is good in this case!
Remember: arches are mainly a result of genetics, and the older you get, the harder it is to improve them. But hey, you're a ballerina, you know the meaning of hard work! :D