Marketing is about identifying the needs of a market and finding a way to serve them better than competitors. If you don't understand your market, how can you create a successful product for them? You can't, unless you're just lucky. And in that case, if you fail to understand the sources of your luck a more attuned competitor may steal market share from you anyway.
If you develop the product without understanding marketing you risk wasting time on something people don't want or that is strategically difficult to grow. For example, a hyperlocal events app faces different challenges to a two-sided marketplace like ebay, to a service company, etc. Even backed up by solid marketing it's difficult enough to create products people want. You can't just sprinkle on some magic marketing fairy dust after the product is built and expect to turn it into a hit.
You also need to understand your competition so you can find out how they're failing to meet the needs of niches. Or if they aren't and they're nailing it, come up with a different idea and move on.
If you're an idea guy, the most important thing to learn is to test ideas. That fundamentally involves marketing, perhaps combined with a prototype/MVP for ideas that pass initial filters. Don't fall into the trap of staying in your comfort zone as a developer unless you can find a marketer to work with who'll be the business brains behind your projects.