I don't think you need a detailed 50 page plan, but you need a general idea of your short-term and long-term goals and most importantly, how you are actually going to make money. I find that this is important because without a list, it's very easy to veer off the path and start going in wacky directions, which can lead to your company never getting off the ground or releasing a new product, etc (this has been my mistake for years)
Many software startups start out as something cool and then the owners scramble to find a business model. If you want a better chance at success, figure this out in the beginning and then mold your cool idea to your business model.