Backpropagation Through Time - Why do you need truncated backpropagation through time?

To train sophisticated recurrent neural networks, such as Long Short-Term Memory networks, researchers employ a variation of the Backpropagation method called Backpropagation Through Time. If you need to anticipate a sequence with a very long sequence, you should use Truncated Backpropagation Through Time, an improved version of this approach.