I don't know about you but here is my experience from my first mobile app:

After 3,5 months of development I was really looking forward to launching, relaxing and observing. I still remember when I received the notification that my app is selling and it didn't take long until the first sale came in.

But then you just realized that once your app is finished and launched, you are just starting. Marketing, sales, letting other people know about your app/startup, bug fixing, customer support.

While marketing is not everything, you will realize that even if you have an awesome product you have to market it because there is simply too much out there that anyone will notice. Of course if your product is not great, your marketing efforts are wasted. But without marketing efforts, nobody will know about your app.