As a project manager
As a project manager, you do not know every process that goes on in a company – especially when it is a large one. It is always the best option to talk to the people who do jobs/tasks every day. They are the ones that have worked on projects and tasks and are familiar with it. They will know how long a task will take and can help a project manager in better understanding the task –thus giving project manager a good sense of how to estimate time and budget.