Why (if anything) is a planet clearing its neighborhood important?

[Tvtyrant]

I'd gotten the impression that said splashed debris contained the greater part of Theia. That the moon was essentially Theia after being melted and dispersed by the energy of the impact, exchanging some material with the outer lat=yers of Earth which was also partly melted by the impact, losing a bit of its mass to Earth and to deep space, and then finally recaolescing and resolidifying.

I don't think Theia melted, I think it go vaporized. It was only 1% of the size of the object it impacted. The moon is likely made of ground that was a decent distance away from the impact that kept its crusty form, the actual impact site and Theia turned into a cloud that then rained down on the planet for a few years.