What would have happened if Earth had not collided with Theia?
It would’ve made a huge difference. Why? Well, first of all, the moon is actually the celestial body that remained from Theia. But that’s not so much the biggest difference.
When Theia collided with the Earth, it caused the Earth to be slightly tilted, causing us to have seasons today.
Life getting to existence might have been very difficult without these seasons and the materials Theia brought with it.
Without that collision we might not have been here today.