A notching tool is used to crimp the rails so they lock into the posts. Most rails come notched on one end (on occasion both ends). The only time you would need a notching tool is when you cut a rail to shorten a section, or if you want to stagger rails, or when you add a gate. For smaller enclosures, people are inclined to cut rails short instead of bending them, which makes for a chopped up look. The rails are designed to bend, and generally enclosures look better when rails are used in their full length.