Side tabs stay visible along the edge whether the document is lying flat or sitting upright in a binder, so you can flip straight to a section without opening the thing first. Bottom tabs make more sense for documents that lie flat and get read top to bottom, like certain accordion style files. For most binder based numbered systems, legal exhibits, mortgage packages, big reference binders, side tabs are the more practical pick because they stay visible even with the binder closed and shelved.