Knowing how to sell/market your product is more important than having a fully functional or carefully engineered product. Nobody will give you money for your (master)piece of software if they don't know what it does or what it's for. Sounds trivial, and yet the better you are at software development, the harder it is to keep in mind.
Just because you are good at something doesn't mean you should be focusing on that thing and leave the rest for later.
Just because you are good at something doesn't mean you should be focusing on that thing and leave the rest for later.