Sometimes traditional advice is very bad advice. "Write a business plan before launching your company" falls firmly into this category.
After all, battle plans don't survive the first five minutes of contact with the enemy, and business plans are a lot like battle plans. This is why I've run multiple successful companies without ever once developing any kind of long, drawn-out business plan. It just didn't take me very long to discover that almost everything that I thought was going to happen was not going to happen.
Nevertheless, I did at least try to develop a traditional business plan when I started InternetReputation.com. My partner and I did our research, looked at our potential competitors, and put it all down on paper. We were absolutely sure we knew exactly what we were going to be doing. I had ideas for the customers we'd target. The prices we'd set. All of the things we're all supposed to be writing into these plans.
Two months into the business, we realized that everything we'd planned for just wasn't working. We were going to have to change our prices. We were going to have to adjust our target market. In fact, we pretty much had to change everything there was to change about the business as we'd planned it.
If we hadn't been ready to toss out our plan, we'd have failed--as would any business that proves itself unable or unwilling to adapt to the ever-shifting conditions of our volatile marketplace.
A business plan is just a bunch of dreaming and speculating on paper. Investors (if you have them at all) aren't even necessarily going to read it. You just have no way of knowing how things are going to play out until you're right there in the trenches, battling for the life of your business.
Sure, you do need to decide a few things in advance. Obviously, you want to know what you're selling. It's also a good idea to at least have some notion of who might buy this thing, and how you might reach them. As for prices--well, you have to start somewhere. But a 30-page plan full of detailed fantasy projections of revenue that isn't here yet isn't going to help you achieve success.
There is only one thing that will help you achieve success: the ability to creatively solve problems as they arise. If you're not preparing yourself for the possibility that you will face huge changes during the course of this journey, then you are setting yourself up for failure.
Is a plan ever useful? Later, yes. When you've put in 2 to 3 years of business time, and you actually have some idea of how your business works. Then you can create a plan for growth, one that is based on actual numbers. Even then, you'd best stay on your toes: There's no telling when you'll have to throw that plan right out the window.