Why are costs estimated for individual jobs?

The reason for estimating the costs of either one job or many jobs is to provide a forecast for the organization filling and/or creating new jobs to see if the organization can afford them. If the job (s) are already in existence, then the answer is to provide a forecast for what the costs will entail at some future date.

The items to be estimated are numerous and actually the items themselves are irrelevant to the question.