A life plan is simply a map for your life.
As you move through life you know more about your life and your environment and so you adjust your map.
When you use a map you are expecting to be lost otherwise you wouldn’t need a map. You keep a map in your car (or have google maps on your phone) for those times when you get lost.
If you just meander you will miss opportunities life has to offer. Having a map increases your chances of getting to where you want to go. Each time you break your plan it is because you have learned something about the journey towards your goals.