Flat brushes have a square end with medium to long bristles. They are great for wide strokes where you want a lot of paint coverage. If you are working on a surface where a wash brush might be too large, the flat brush gives you excellent coverage while also providing you a little more control over the wash brush.
Flat brushes can also be used for thick to thin lines, however for many people the angled type will likely provide a little more comfort and control when working on a upright cake.