Simple to say, probation period is like a free trial.
It is compulsory for an employer to guide new employee to adjust to their work. Even you install software also need to choose location to install.
If the trial period end, you feel the employee not suit to the job, you can giving a notice of termination to the employee. Like uninstall a software but u need to wait the uninstall.
If you feel maybe the employee can adjust to the position but he/she need more time. Then you can extend the probation period (optional). You can apply for another free trial.
But remember, after the free trial if you don't want it. It may cause some bad image to the user whereby the employee may say bad thing about the company.
I hope by using Pc as example, u can understand clearly.