According to this instruction, every living being will go through suffering in life. Disease and death are inevitable and so is emotional pain. There is a belief in reincarnation and the Buddha says that we should all know that in our future lives we will go though suffering. If we can understand this, then we can use our present life to get liberation from future sufferings.
On the other hand, if we do not realize this simple fact, then we just might spend our present life trying to attain happiness for this one life, instead of freedom for the countless lives that follow.