what is different between wmi and API

i work with c++ programming, I use an example for understanding main of my question.

Suppose, we want get current username in windows operation system, we can use follow code :

#include <windows.h>
#include <Lmcons.h>

char username[UNLEN+1];
DWORD username_len = UNLEN+1;
GetUserName(username, &username_len);

also, we can use wmi by follow the instruction explained on here and use Win32_ComputerSystem.UserName .

so, I hope you have fully understood, what's different between wmi and using api or any other way?

tank you for your response.