For example some employees can override many checks and procedures for operational reasons and if they want they can use customer funds or conspire with people from outside the organisation to do shady things.
On a normal business day people will coordinate all the time and funds or resources can be moved, locked, released etc for operational reasons and no one will question a thing because everyone knows their part only and those who know the big picture are not exposed to these operations on daily basis.
So if an employee is doing something shady, they will act based on the developments on the ground. For example if a client is going to withdraw their money, they can move funds from another account to cover for the missing funds. If the employee is cut off for an extended period, they won't be able to arrange this and will trigger an investigation.
There are of course regular audits but these audits will catch you only if funds go missing or some other obvious irregularities are happening because they can't analyse every single action of the employees.
I was told that it's not unheard of employees taking the money from the client accounts and invest it stocks or crypto, take the profits and put back the money. They are usually caught when the investment doesn't pay off and can't put the money back.
Also, apparently there are cases of employees conspiring with criminal organisations to launder money. Handling of the client by another employee can reveal the shady business.
So in essence, any fraud that requires maintenance can be potentially uncovered in the absence of the fraudster.