Why did Jesus use the expression, "very truly I tell you"?

I used to think nothing of it until someone said, *"it makes you wonder what he was saying the rest of the time"*. Now I don't believe for one moment Jesus was a liar, but why did He use that strange expression? One of [26 examples](https://www.biblegateway.com/quicksearch/?quicksearch=%22very+truly+I+tell+you%22&qs_version=NIV) exists here: >[John 16:7 (NIV)](http://www.biblegateway.com/passage/?search=John+16%3A7&version=NIV1984)
> But **very truly I tell you**, it is for your good that I am going away. Unless I go away, the Advocate will not come to you; but if I go, I will send him to you.