When they do not follow the original plan, it means they started to get an onjective but instead they faced with other hurdles and hence changed the paths. This means their learning from their own mistakes is faster. They know what will work and what will not.

They can figure out what is to be ruled out, what new direction to look forward to$.$

But at the same time there are learnings from the original plan, strategy and vision that is used.

The greater the detail the more they can adapt from the original plan. So $,$ it is always useful for the companies to have a strong business plan, even if they do not fully adhere to it$,$ the more detailed or analysed it will be the more the founders can use it even when it is completely changed