Ask HN: Are business plans really not useful?

As hackers and entrepreneurs, we read a lot about how business plans are useless and all that stuff. Cool thing to do is start building stuff and iterate/pivot. But are there scenarios where they could be useful ? Anyone here has used business plans in any way that helped ? When i say "business plan", i m not necessarily saying the traditional 100 page doc. Just interested in understanding if people write stuff up specifically for potential investors ? Or is it plain "I built this. This is my plan. I don't do documentation"