I am still very much learning about startups and building a business but the bottom line is: whatever your product or service, there need to be customers for it who are happy to pay the price you ask for. This is one of the details you have to think about thoroughly when writing a business plan. This in turn forces you to really understand your customer and their particular needs so that you can make sure your product or service is meeting these - not just your own ideas of what a great product should look like.
It does not matter whether you are selling to the public, private or third sector, whether it is a luxury item or something to "improve the public good". Your product/service needs to be affordable by the user and do the job they require it to do. The business plan forces you to work out these details.
To many engineers like myself this kind of work compares poorly to working on the tech side of a soution. But it is a VERY useful exercise and it can actually be fun once you get into it. Having said that, the infamous business plan is also often a document that is created at great effort for someone else (like a funding provider) and then filed, forgotten and never updated. A wasted effort really.
To get started try something much more visual like drawing up a 'Business Model Canvas' and 'Value Proposition Canvas' (see https://www.strategyzer.com/ ). Consider it a precursor to the business plan and "something you can pin on your office wall" to keep it updated more easily. There are plenty of useful short tutorials on the above website (just need to register for free) to explain the concept. It is making a lot of sense and I found it very useful in getting the business basics sorted out.