The two reasons people order the extra heads is for multi-use and sanitary needs. It's helpful to keep one head near the toilet to use strictly for toileting and one in the shower (or on the counter top) to use for other hygiene needs. It's also helpful to have an extra head on hand for a second bathroom, for work, a second home, travel etc.
FreeedomWand® Head Replacement:
Use one head for toileting and one for grooming. If you don’t need the extension, purchase a head and handle to keep your costs down.