It used to make sure, kinda.... Let me give an example: 먹다 = eat. 밥을 먹어요? = Are you eating rice? 밥을 먹지? or 밥을 먹지요? = You're eating rice, aren't you? Short of :D Hope this helps!

As Above You Will Also See It Before The End Of Things As A Part Of 않아 like 그쪽과 점심을 드시고 싶지않아요. by which the 지 is there as like a buffer between the 않 and 싶 ~~~ i think this way is the way it is used to negate the end of regular verbs like above..