The flat brushes are good for different techniques, depending on their properties.
You can use the flat brush also for covering the surface and for creating thick to thin lines, You can make wide strokes, you can create leaves, flowers, you can work with it on trees, grass.
Also you can use your flat brush to blend the colors
Here you can find a few tutorials on how to use your flat brush to get the most out of it.
Some of them have been very useful for me too, when I started to paint, as I learned a lot from YouTube tutorials.
Thank you for your beautiful and very useful training materials !
How to load your brush
How to make 8 different flowers with your flat brush