Most watercolor artists use round brushes for at least 80% of their work as it gives them great control over the painting process. The flat brush is usually used for washing big areas or buildings and not an all-around brush for most artists.
I believe flats are a great tool for becoming more loose and impressionistic with your work, focusing more on connecting shapes, tone and establishing a rhythm rather than focusing on details and coloring inside the boxes.
So I’ve started using them for most of my recent work and there’s still a long way to go. Here are some of the examples: