The water needs to he hot enough to extract the optimal flavour from the tea fast enough. If you leave it longer, the bitter, oil based flavours start to come out excessively.
This was actually a noted problem for Victorian (British) explorers. They couldn't brew a proper cup of (black) tea up a mountain! I believe there were also patents issued, at the time, for tea brewing pressure vessels!
Basically, you want the water as hot as possible, but not so hot it scolds the leaf. Black tea doesn't scold (at normal water temperatures), so is brewed at 95°C+. Green tea scolds at around 80, so needs cooler water. White tea is even lower, again. By around 60°C your into stewing temperatures. You'll get a lot more oils compared to flavour. You want to have your tea leaves out by this point.
You also want to avoid agitating the leaves. Stiring, or squeezing the leaves tends to ruin the flavour. You know you've completely F'ed up a cup of tea if it has an oily scum floating on the top.