Right now, when you have many folders, you can of course name them and know what's inside them. However, some people have folders in their bookmark bar and give them a blank name so that only the icon is visible (allowing for more room in the bar). This may be annoying for people who put multiple folders in their bookmark bar and cannot tell them apart. Plus, I would just like to have little icons for my folders instead of them all looking the same.
By "customizable", I just mean that you can make the icon of the folder the same icon as any of the ones in the folder itself. However, I think it would be fun if you could get a small set of pre-made icons specifically for use in decorating folders or even using custom images (although I'm not sure if that would work too well). the point is we ought to have options to make each of our bookmark folders visually distinct from one another.