The Quran is the Best Tafsir?

In S 25:33 we read: And they do not bring you a similitude but we have brought you the truth and the best tafsir.

If the Quran is the BEST tafsir, of itself, why do Muslims go to Ibn Kathir, et al.? Also, did Allah (“we have”) come to Earth and give it to its boss? Or is Allah permanently on Earth? (Side note: Does S 55:26-27 indicate that Allah is on Earth?: Everyone on it is evanescent. But the face of your lord, the possessor of majesty and honour, remains.) If it refers to angels, where does it state that? If ‘we’ can refer to angels, how might we know when it refers to angels and when to Allah?