A business plan may or may not be a legal document. There isn't any kind of standard for a business plan. It can range from a single page to hundreds of pages.
It is considerable and important when:
You want to Sell out your business:So here from the buyer's perspective, it is important.
In search of pursuing Finance for your business: if you want to seek more finance for your business then a business plan is an important document as it administers and clarifies more about your business.