One way is to see that such Buddhas (Bodhisattvas too) exist is as a paradigm, an ideal, a goal representing the best of the human condition to which men and women can aspire. As I said in a talk last week on Kannon, the symbol of Compassion: It does not matter that she "may not be really real", for we make Kannon "really real" in life:
I had a hard time, for many years, incorporating into my practice many figures such as Kannon and Jizo ...
I have come to see "them" as archetypes, representing real characteristics of human life and (since we are just the universe) thus the universe.
In other words: When we feel in our hearts and act upon Love and Compassion, thereby Love and Compassion exist as real, concrete aspects of the world which our hearts and acts create. There is no "inside" or "outside" ultimately, thus what is inside you is just as much "the universe" and concrete reality as the moon, gravity and the stars. That is "Kannon", in that way a real and concrete aspect and 'force' of the world. Her 1000 helping hands are our hands, and our actions make her real in the world.
As with Bodhisattvas, so it is with the Buddha, all the Buddhas. Wisdom and Compassion realized in each of us is the realization (meaning both "the discovery" and "the making real") of Buddha in the world. We make Buddha real, Kannon and the other Bodhisattvas too. (Mara and the Devil too if we act badly).