Employers do not like their employees to have criminal records. It's why they ask you about your criminal history on job applications. It's why they deny employment to candidates who have criminal pasts.
It comes down to one simple thing: Liability.
Employers believe that it's a liability to hire people who may have made mistakes in the past.
Employers worry about their own workers stealing from them. They also worry about what could happen in interactions between employees with criminal records and customers. What if it's a business where employees go to customers' homes?
If an employee commits a crime against a customer and the employer knew or should have known that the employee had a criminal record, that employer could be held liable in civil court for any act committed by his or her employee.
When you seal or expunge your criminal history, you may be able to legally deny that you were ever arrested.
There are a few agencies that are entitled to sealed or expunged information.
Most private sector jobs will not have access to a sealed or expunged arrest. If you have only been arrested once and you are eligible to have that arrest taken off of your record, you should. It may be the difference between getting a job and getting denied employment.