I cant understand what is the difference between the xpath with inside position method and make a t-call in a t element and later add some code inside it.
xpath is used to make the view of odoo backend. t-call is used to make the view of odoo fronend.
Now if you add an element with XPath & position inside that updated view effect will be shown when its parent view is getting loaded.
With qweb t-call the view you are calling will not change & will not show your changed effect if you call the main template on any page, to see your changes you must use template id in which you are are using t-call & making changes.