Everyone is clear on what the milestones are and when
Everyone is aware of the dependencies and what other work is happening when
You are able to clearly track progress
You are unlikely to miss something major by having a plan
Disadvantages:
Too much time can be spent tweaking Gantt charts to get everything lined up
Too long of a plan takes a great deal of time to update
Project sponsors assume every task as set in concrete which reduces your flexibility to change the order of deliverables around and yet still meet the end date
Getting bogged down in the details of the plan can cause one to lose sight of the big picture
Increases the risk that time is spent managing the plan instead of managing the people
