This punctuation mark is well known and like the period, it means that a thought has been completely expressed, but it also tells the reader that the writer has not expressed the sentence or thought as an assertion but is asking something.

"Jim fell down." "Jim fell down?" The first sentence means one thing; the second means something quite different.

Since the initial capital letters and the three-worded thoughts are exactly the same in both sentences, the differences in meanings are communicated only by the different punctuation marks at the ends of the two sentences and these differences can be communicated in no other way known to writing and printing.

Why is a question mark?
What can it be?
Already you've two of them. How about three?
It looks like an acrobat perched on a ball
Who has to be nimble or else he will fall.
His legs come down straight,
But his back is a curve,
And keeping his balance
Takes talent
And Nerve.
Or is it like smoke that comes lazily curling
From a blaze underneath in a ball that is twirling?
Or a hook used for hanging?
Or maybe a genie
Coming out of a bottle?
(The bottle is teeny.)

Whatever it looks like (have you a suggestion?),
The question mark raises (and lowers) a question.
No reason to scorn it or ever to doubt it.
This mark's made its mark.
Any question about it?