The basic purpose of is to put at the end an exclamatory sentence, such as one where we are in disbelief (ex. You did what‽)
It is a very uncommonly used symbol as most people would simply end their exclamatory questions with both of the symbols like this: !? or ?!
On a computer an interrobang looks like this: ‽
On a chromebook the unicode is: Ctrl+Shift+u+2+0+3+D