proper planning is the most important part of any project, you keep track of the most important resource i.e. time. weekly milestones help with this, you know where you want to be at the end of this week and therefore you know how to spend the time you have. this way you are not shooting arrows in the dark and have a roadmap.
planning is not easy, you can over estimate your abilities and add too much to your plate. so having a reasonable free buffer is also important so that you have some time to be creative with your approach, some wiggle room for possibilities.