Filberts are like flats with rounded sides. The filbert is a good brush to reach for when you don’t want the sharp edge of a flat.
There are also variations on the filbert. One of these, and I’m not making this up, is called an Egbert.
This extra long filbert will hold a lot of paint, or when used dry, it’s a good blender brush. I have a few that I use primarily for drawing with thin inky paint.