Say that you have recently promoted a shop assistant to the position of manageress, but they don't meet your expectations. While the employee worked well at a lower level, they clearly struggle with more responsibility. This is where the "trial period" comes in.
Where a member of staff is promoted, a trial period clause is useful. If they don't meet expectations or it is clear that they are struggling in their new role, you will be able to put them back into their old job. In other words, their promotion isn't permanent until it is confirmed and you reserve the right to revert them to their original role.
You can set a period of three to six months for them to be assessed in their new role. In both cases the employee knows his or her employment is being assessed and is subject to confirmation, so this should mean they work harder to prove themselves.
Finally, if there is a problem, act before the probation period ends. If you miss this by just one day, employment is automatically confirmed and you cannot rely on the clause.