Why do recruiters ask candidates for their expected salaries if they already have a certain number or an offer in mind?

I agree with the point that recruiters want to know what insists the person to demand such salary.  in my opinion,  the employers are trying their maximum to get highly expereinced and qualified employees with minimum salary.

Recruiters want to know what motivates the person. If a person says a money, the recruiter should dig a little deeper, specially when a person has some length of seniority at the position being left.

Companies ask for salary history to give them an advantage in negotiating the salary they're going to offer.

To know if the candidate is in the company range budget. 

They don't know yet, but this is essentially a chance for you to invest in learning this skill: negotiation 

If you came to an interview expecting a certain amount, it's not on that day that you made a decision, it's months or years prior to that interview.

A recruiter must tap into your expectation, sine they have an expectation, if the expections don't meet it's best for both recruiter and recruitee to not move forward.

However an offer can always come your way with their expectation to see if you will accept, this is where negotiation starts. Though they know you have an expectation they still offer from their expectation-view, because you are the right candidate; hence build up on negotation skills.

i work as HR officer never ask any employee about the expected salary iam always tell them about the company offer

To Measure if the candidates are in company budget range.

tHEY WANT TO KNOW  WHAT BENEFITS THEY WILL GET FROM THE CANDIDATES .

To offer a competitive offer

It is to know how much they are expecting and decide appoint them according to what they ask as salary.

I think he wants to know the person's confidence in himself in terms of appreciation of the value of the .salary