mongoDB not inserting value using angular factory to do post method - angularjs

Question

how come its sending data to mongo if I $http.post directly in the controller?
$http.post('/api/users/', vm.newUser).success(function(response){
But using the code below doesn't seems to work, it's not passing any value but generates ID in mongoDB
vm.signupNewUser = function () {
signupService.signupNewUser(vm.newUser).success(function(response){
console.log(vm.newUser);
}).error(function(err){
console.log(err);
})
}
service:
function _signupNewUser(username, password, email, fname, lname){
var data = {
username: username,
password: password,
email: email,
fname: fname,
lname: lname
}
return $http({
method: 'POST',
url: '/api/users',
data: data
})
}
Edit: this seems to fix it, but not sure how is it different from using a direct $http.post and just passing vm.newUser. But using factory I have to pass in vm.newUser.username and etc. I don't think passing the vm like this is ideal but please correct me if I'm wrong.
vm.signupNewUser = function () {
signupService.signupNewUser(vm.newUser.username, vm.newUser.password, vm.newUser.email, vm.newUser.fname, vm.newUser.lname)
.then(function(response){
console.log(response);
console.log(vm.newUser);
});
}