The Buddha in his wisdom can affirm that the world is already saved and pure. As the Lotus Sutra says, “Tranquil is this realm of mine.”
Nonetheless, in his compassion, since the world is full of beings who groan amid miseries of every kind, the logic of love draws him to reveal the Dharma to people devoid of wisdom and full of attachments:

The Triple World is not safe,
Just as the burning house,
Full of all kinds of sufferings,
Was greatly to be feared.
Ever there are the distresses of birth,
Old age, disease, and death;
…
Now this triple world
All is my domain;
The living beings in it
All are my sons.

But now this place
Abounds with distresses;
And I alone
Am able to save and protect them.

For this reason the buddhas appear in the world:

Because the buddhas, the world-honored ones, desire to cause all living beings to open [their eyes] to the Buddha-knowledge so that they may gain the pure [mind], [therefore] they appear in the world; because they desire to show all living beings the Buddha-knowledge, they appear in the world; because they desire to cause all living beings to apprehend the Buddha-knowledge, they appear in the world; because they desire to cause all living beings to enter the way of the Buddha-knowledge, they appear in the world.

For this reason Śākyamuni, the Eternal Buddha, says he is committed, for a time without end, to bodhisattva practice.
A Buddhist Kaleidoscope; Riccardo Venturini, A Buddha Teaches Only Bodhisattvas, Page 334-335