When should I use user_load($user->uid) when do we need to load user using  user_load($user->uid) after global $user. Why do we need to load a user because we can access user data after declaring it global. I am adding a commerce customer to his billing and shipping profiles, should I use user_load?
The global $user object is just the bare data received from the users table for the associated user. It doesn't have any field, or other module-added data, attached to it by default.

For example, if you have a field attached to your user accounts called 'field_nickname', and you want to inspect the value of that field for the current user in code, you would need to call user_load() for the data to be available.

// Might work, but it's not guaranteed - depends on the modules 
// you have installed and what's happened in the page build so far
$account = $GLOBALS['user'];
if ($items = field_get_items('user', $account, 'field_nickname')) {
  // ...
}

// Will always work.
$account = user_load($GLOBALS['user']-&gt;uid);
if ($items = field_get_items('user', $account, 'field_nickname')) {
  // ...
}


There are situations when other modules will load the field and other module data onto the global $user object, so depending on what modules you have installed you call to user_load() might not strictly be necessary.

However, it's good practice to make sure you load the object yourself; if another module has already run it for the current user then that data is statically cached, so you don't get a performance hit.