A business plan is what you want to do, written down. It's not an 80 page MBA document with a bunch of pie charts. The sooner you come to terms with that, the better. Write that stuff down, show it to people, and make sure you can explain it properly. Your plan will change as new opportunities emerge, but it's important to always know what you want at any given time, otherwise you won't get it. No one else is going to figure that out for you and hand it to you, so you need to think about it and aim for it.
Do you want to grow your company? How big? Do you want to sell your company? To whom? Do you need investment? How much, from who, and what for? What milestones do you need to reach to get what you want?
Properly defining your goals is a huge part of what your business plan is for, and in my opinion it's the most important part. Something magical happens when you write stuff down - suddenly it's a lot easier to communicate and refine your vision, which really helps keep your company focused.
Your goals can change dramatically based on unanticipated things like the release of new technology, or because of a shift in your business model or customers. Your entire company could pivot. Nevertheless, you need to have definite goals in mind at all times, even if it changes.
Share your goals with good advisors and people you trust. Talk about them. Aside from thinking about how you will get there, think about what you will do after you get there.
In my experience, a lot more emphasis is placed on having a traditional business plan when you're from a small town. Maybe it's because the business community in small towns tends to be from traditional industries where capital comes from bank loans or funding. I have never been asked to show my business plan. It's far more common that people want to talk to me at length to establish whether I know what the heck I'm doing, and they might ask for specific things like my cap table (who the company belongs to - the share structure) and an executive summary.
Having said that, it's still really important to write a plan. If you remember that it is a tool for you and your team, and you'll actually find the process a lot more fun. Skip all the stuff you don't know yet. Don't stress about writing it in a certain style.
The heart of my business plan is mostly in the form of summaries of whatsapp and email conversations with my co-founders, annotated roadmaps, and links to market research and data about competitors. It changes all the time, so I don't bother versioning it - I just add new stuff to it and move things that are on the backburner or potentially being dropped to the back. I keep funny and crazy ideas and notes to myself or from my team in there because if it's fun to read I'm more likely to look at it.
If I ever needed to make a traditional boring business plan the data is all in there, but I seriously doubt it will come up at this point. What is important is that I have a roadmap to keep me focused and on track, and the reference material I need to explain what the heck is going on to anyone who needs to know.