They may feel the need for a written plan to help manage an already rapidly growing business.Or a plan may be seen as a valuable tool to be used to convey the mission and prospects of the business to customers, suppliers or others.
Business plans can help perform a number of tasks for those who write and read them.
If you're just going to use your plan for internal purposes to manage an ongoing business, a much more abbreviated version should be fine.
About the only person who doesn't need a business plan is one who's not going into business.
Most books on business planning seem to be aimed at these startup business owners.
There's one good reason for that: As the least experienced of the potential plan writers, they're probably most appreciative of the guidance.