Death Is Certain
(1) Death cannot be avoided.
(2) Life cannot be extended; it is always diminishing.
(3) Even while we are alive there is little time for practice.
The Time of Death Is Uncertain
(1) Our lifespan is not fixed.
(2) There are many causes of death, and few for sustaining life.
(3) The body is very fragile.
At the Time of Death, only Dharma Can Help Us
(1) Our friends cannot help us.
(2) Our resources cannot help us.
(3) Our body cannot help us.
The Importance of Reflecting on Impermanence
Kyabjé Trulshik Rinpoche explains:
“If we really meditate on impermanence, it is said that in the beginning it can be the cause of us practising the Dharma, in the middle it can provide the conditions for us progressing along the path, and at the end it can cause us to achieve the result of complete and perfect awakening. So impermanence is of the utmost importance.
If we consider the main part of the ngöndro practice (the inner preliminaries), from refuge and bodhichitta through to guru yoga, the meditation on impermanence is important at every stage. It is the greatest help at every stage. …
Being mindful of death and impermanence can help to provide all the causes and conditions for practising the Dharma and gaining the result.”
Source: https://www.rigpawiki.org/index.php?title=Impermanence#Reflecting_on_Death_and_Impermanence
He mentioned it in his first sermon near Varanasi, and almost his last words as he passed into parinirvāṇa at Kuśinagarī were, “Everything, whether stationary or movable, is bound to perish in the end.”
He also illustrated its importance with many examples. In the [Mahāparinirvāṇa-]Sūtra, he said: “Of all ploughings of a field, the autumn one is the best. Of all footprints, the elephant’s footprint is the best. And of all recognitions, the recognitions of impermanence and mortality are the best. They eliminate all attachments of the three realms, ignorance, and pride.”
In addition, “It is also praised as being the hammer that destroys all defilements and misconduct at once, and the great gateway leading to the accomplishment of all virtues at once.”
(Geshe Rabten. The Essential Nectar.)