Why an employee should be promoted?

Why an employee should be promoted?
The various reasons for wanting to promote an employee are:-1. They are very good at their job and you wish to reward them for what they have done. 2. You consider they would do well at the job you have in mind for them to do. 3. They would be stretched and extended by the new job but you see that is necessary to grow them into further responsibility. 4. This is next one not a good reason for promoting someone but it happens - You cannot stand them and they are awful at their job but you know they will be out of your hair and someone else's responsibility if they go to another job within your organisation.