Why we use Question Mark in Regular Expression

This ? is used as follows:

? Match zero or one of the previous expression

"The previous expression" means the character that comes right before the question mark.

Let's say that you have an economics website and you only want to look at the referrers that have the word "labor" in their title. But some of those referrers come from countries where they spell it "labour." You could create a filter like this: labou?r

That way, it will match "labour" (which does have a "u," which is the previous expression) and labor (which has zero of the previous expression, i.e. no "u" is included.)

You cannot use it like this : labo?r, or at least, not for the same purpose. It's not a wildcard that you stick in between the o and the r to match any letter. The only matches would be to "labor" (zero of the previous expression) and "labr."
Found it here[^]

Quoting from the help-file of Expresso,


Repetitions
You've seen that "{3}" and "*" can be used to indicate repetition of a single character. Later, you'll see how the same syntax can be used to repeat entire subexpressions. There are several other ways to specify a repetition, as shown in this table:

*	Repeat any number of times
+	Repeat one or more times
?	Repeat zero or one time
{n}	Repeat n times
{n,m}	Repeat at least n, but no more than m times
{n,}	Repeat at least n times