The liner brushes are good for different techniques, depending on their properties.
They are best used for fine details in your artwork, as you can have a lot of control of the paint brush and you can create very thin lines.
You can even use them for lettering and adding words to your pages.
Here you can find a few tutorials on how to use your liner 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 !