Why do questions end with question mark?
To signify that it is a question.
In speech, we want people to understand what we are saying - otherwise it is rather pointless to say it.
The question mark (along with other punctuation) makes it clear about what we are saying and how we are saying it.
- He did that today
#color(red).# #"vs"#He did that today #color(red)?#
Here is the same sentence, written with a question mark and a period. Can you see how we would read these differently even though the words are no different?
As you can see, question marks help to evoke certain emotions, and help people clearly understand when you are asking a question.