Confusion regarding Mongo db's CRUD statements

I started learning Mongo db today and am little confused about following update statement.

var collection = db.collection('test');
var doc = {mykey:1, fieldtoupdate:1};

collection.insert(doc, {w:1}, function(err, result) {
collection.update({mykey:1}, {$set:{fieldtoupdate:2}}, {w:1}, function(err, result) {});
});

My specific questions:

1. Why are we calling update inside insert? Isn't insert supposed to be used for inserting a new doc in the collection?

2. Can't I simply write following instead of writing it in the callback of insert?

   var collection = db.collection('test');
   var doc = {mykey:1, fieldtoupdate:1};
   collection.update({mykey:1}, {$set:{fieldtoupdate:2}}, {w:1}, function(err, result) {});
   
   

via nitinsh99