So I have a theory that was spurred on by this tweet by Aster1: https://twitter.com/Aster1/status/1268864445788364801?s=19

Well, the first thought when I saw this was, we've always assumed the Traveler that visits the planet Fundament, abandons the Eliksni during the Whirlwind, and then comes to US are all the SAME Traveler. My thought is, what if they all were one of a species of beings like the Traveler; my thinking being that not only does it never explicitly state they are the all and the same Traveler, but there are also seemingly hundreds of Pyramid ships, so why should there be only one Traveler? Also, what if the reason the Traveler defended humans and the Sol system is because it used its paracausal abilities to predict they would be allies in vanquishing the Darkness?

The Krill betrayed the Traveler and Leviathan to the Worm Gods and became the Hive. "Communicating with magnetic fields, it attempted to persuade them to return to the surface and follow the way of the Traveler and the Light, warning them against the seduction of the Deep, but the sisters rejected its (the Leviathan) pleas because it offered no way to end their species' suffering and avert the Syzygy." From Dentinypedia, about Fundament having been home to the Traveler and why Aurash, Sathona, and Xi Ro betrayed the Traveler

Meanwhile, the Eliksni fell into period of civil war and became pirates and mercenaries before eventually pursuing the "Great Machine". On the aftermath of the Whirlwind: "In the aftermath, the surviving Fallen became embroiled in a bloody civil war over their civilization's ruins, before achieving a tenuous peace and embarking on a journey into space to locate the Traveler in order to return to greatness." So they didn't go right after the Traveler, they fought for an unknown amount of time before pusuing it. So if there are multiple Travelers, they may have stumbled upon the one from Sol and thought it was thiers.

The Traveler stayed and nearly sacrificed itself for humanity because it knew humanity would protect it and join the fight against the Darkness with enough time. The Light shockwave that destroyed Ghaul wasn't just to destroy Ghaul and free itself, it was to let other members of the Traveler's species know where it was, that they they had finally found a worthy ally in the fight against the Darkness. Basically, a calculated risk to summon aid before the Darkness could arrive. I think Eris and Drifter are finding a second Traveler.

The reason I think the Traveler isn't the Light itself is because if the Traveler is the physical manifestation of the Light, I don't think the Heart of the Black Garden would work as described in every piece of lore and description of lore I've seen. The Heart was supposed to be blocking the Light from healing the Traveler, but if the Traveler IS LIGHT, how can it block something that that the Traveler already is made of? I think the Traveler is a servant/follower of the Light, like the Pyramids are servants of the Darkness.

TLDR: The Traveler saved humanity because it finally found an ally, and it is not Light incarnate, but a servant of the Light, and there potentially many of them just like the Pyramids. Eris and Drifter are going to find another Traveler on Europa.