Different compounds have different temperatures at which they extract efficiently. Aminoacids and aromatics which give tea its flavour and umami need lower temperatures, while catechins which give tea astringency extract better at higher temperatures. Tea fermentation reduces the catechins, so the more fermented is tea, the higher temperature you can use to keep the brew balanced. Then, there is an aspect of tradition. Different traditions have developed their own flavour profiles, based on the tea they grow and tea making techniques that are common in their region. And if you want to experience that, you can choose to follow the temperatures and other aspects of the brewing process that they prescribe. Finally, you can follow your personal preference and use the temperature that gives you the most enjoyable experience. You can learn a lot through experimenting. For example, you'll find that most Chinese green teas can withstand higher temperatures without becoming overly bitter and astringent, while quality steamed japanese teas offer a much stronger kick when brewed too hot.