also, all prophets and messengers were sent to spesific people and every people had a prophet "ولكل قوم هاد" and for every people is a guide (13:7). however, the last of the messengers, Mohammed Ibn Abdallah PBUH, was sent to all humanity, as the final messenger with the final book, the Quran.
angels did not understand the wisdom behind creating this new being, so, they asked why? Allah SWT told them that He knows all, better then what they or anyone else knows.