Follow-Up means an attempt to ensure whether the appointees accept the job offers. if the appointee is already doing a job that is better than the job offered rejects the job offer. So it is necessary to know whether the job offer is accepted or not. It is important to get this knowing in advance. reason is that to appoint another suitable person to the job vacancy without delay if a certain appointee rejects the job offer.
This rejection may happen at the selection step/ interviews too. Some applicants receive another opportunity for the higher demand organizations an at that times some are giving up the previous chance they had. At that times this rejections are happening mostly and some times applicants may not agree to the terms and conditions of the organization, salaries and other benefits that are mentioning at appointment letter. Then if the both parties couldn't solve that initial conflict, then rejections may happened due to the lack of negotiation skill.
If the appointee accepts the job offer, then he/she can appointed to the vacancy.