Yes you are right there is no need to use mailmerge field in my example code.
My requirement is i have template document with some placeholder where i can put the dynamic data. For example i want to add User name and address.

So i need a place holder to know where is user name in the document that i should replace with real user namer.

There are multiple ways to do such use some sort of tokens and just replace such as ###USERNAME### or as you suggest bookmarks etc.

Unfortunatly first example i found at internet regarding my requirment is with mailMarge so i used that.