In the Arya Golden Light, the Holy King of the Sutras, it says the following:

The Destroyer Qualified Gone Beyond (Bhagawan), the Mighty Shakya (Shakya thubpa), Gone As it Is (Tathagata) took birth, born at the time of human beings in the world experiencing five aspects of degeneration with a lifespan of 100 years. In order to benefit those sentient beings who had lower faith, smaller roots of merit and devotion, and who were looking at the self, sentient beings, life, being alive, and the person as truly existent, looking at things perversely; those who were holding onto the “I” and mine; the underdeveloped beings who believe in eternalism and nihilism; and also those whose religion has no foundational steps (anti-Buddhists), in order for them to generate the right view and quickly achieve peerless full enlightenment, that Tathagata, the mighty Shakya showed a short lifespan of the holy body.

Also, aside from that, child of the race, by that Tathagata showing passing away into the sorrowless state, sentient beings generate the recognition that the Tathagata Buddha is rare and very difficult to meet and they generate the recognition and so forth of sorrow and suffering, and then these sutras taught by the Buddha Bhagawan will be quickly preserved, kept, read, and fully understood, and also there will be no criticism about extensively revealing them to others. Therefore, the Tathagata has shown a short lifespan of the holy body.

Furthermore, if the buddhas always abide and do not pass away into the sorrowless state, sentient beings will not generate respect and will not even have awareness that the Tathagata is difficult to meet, and therefore the holy profound sutras taught by the Tathagatha will not be preserved, read, fully learned, and also will not be purely and extensively revealed to others. Why? Because that sentient being believes that the Destroyer Qualified Gone Beyond One always abides in this world.

Child of the race, for example, anyone who sees that their parents have many jewels will not have any recognition that wealth is rare and wonderful. Why? They will recognize their parent’s wealth as existing all the time. Like that, if the Buddha Bhagawans do not always pass away into the sorrowless state, the recognition that they are rare and difficult to meet will not occur.

Child of the race, aside from that example, when anyone whose parents are poor and have no wealth goes to the home of a king or a great minister, if that person sees many stores, wealth, jewels, and so forth, that person has the recognition of this as being rare and difficult to find and attempts to obtain the means for seeking wealth. That person will abandon being poor and destitute and will wish to achieve happiness and wealth.

Child of the race, like that, the Tathagata is seen passing away into the sorrowless state to generate the recognition that a Tathagata is rare and difficult to meet, and of sorrow and suffering. The Bhagawan Tathagata descends into this world only a few times in countless eons, just as an udumbara flower appears only a few times in this world. Sentient beings recognize that it is rare and difficult to find a buddha and they generate faith in the Destroyer Qualified Gone Beyond (Buddha) and also generate the recognition to complete, preserve, and not criticize the sutras taught from the holy mouth. Child of the race, therefore, due to this cause and condition the Tathagatas do not abide in the world for a long time and quickly pass away. Like that, with perfect skillful means, the Tathagata ripens completely the minds of sentient beings.

Also, in the Golden Light Sutra it says the following on the benefits of relics:

Also, the activity of passing away and leaving relics in the world in other time periods shows the skillful means (of Buddha). Sentient beings who make offerings and also who offer service to the relics should look at them as due to the Tathagata’s compassion and root of virtue. That being who makes offerings and offers service to the relic will completely abandon the eight non-freedoms and will meet a Tathagata, will have a virtuous friend, will not give up bodhicitta, will increase his or her merit, and will quickly go beyond from samsara. This is one of the holy deeds and one will not get lost through bondage in samsara. Therefore, you should practice precisely and without distraction, making offerings to relics and offering service.