The planning phase is the foundation of the entire project. It really is how you will execute. If you don’t plan a head, you are bound to fail at some point during the project. The planning phase will determine how the project can end up. There are many things to consider like you stated budget and completion dates, etc. Remember a lot of money goes into these projects, and the company is relying on the team to execute a game plan to make sure these projects can be a success. Yes there will be certain fall backs that will occur, however planning a head and coming up with contingency plans can help cover up from any falling backs and team might have.