A friend visited my studio recently, and when she saw my work area, she asked "Why do you need so many brushes??". Why indeed. That's like asking a woman why she needs so many shoes.
To pursue the shoe analogy: you have some shoes for evening, some for day, some for country walks, some for city sidewalks. In painting, you have some brushes for fine line work, some brushes for heavy impasto application, some brushes for smoothing. I paint in many sized formats, ranging from those you can hold in your hand, to those you can hang over your mantel. Different sizes of brushes are needed to work on such different sizes of substrates. On a large canvas, you want to be able to cover the territory energetically and efficiently, which requires some large brushes.
Brushes also come in different types, based on the shape of the bristles. "Flats" make nice, hard-edged shapes. "Filberts" make rounded shapes. And so forth. A good painting involves a variety of lines and brushstrokes. It's hard to create that variety without different kinds of brushes. It helps to have all weapons ready for battle!
And finally there's the issue of wear and tear. Brushes don't last forever, so it's helpful to have backups at hand. Certain types of very small brushes are particularly short lived. Check out the photo above, showing the short life cycle of a "0" sized brush. It doesn't take long to go from a few short hairs down to just a nub. The more you paint, the faster they wear down. It's a peril of the painter's life!
