You need them to register a birth certificate, to marry, to open a bank account and even to receive a parcel. You might say the hanko validates every official occasion in Japan.
Hanko, or seals, were formally introduced to Japan in the 8th century, in imitation of the established Chinese practice. With the rise of Zen Buddhism around the 12th century, the practice of using hanko was common among monks returning from China. As powerful daimyo began setting their personal seals to official documents issued within their domains, the use of seals became a national custom.
Before the Meiji Restoration, the use of seals was limited to the upper class. That changed drastically in 1887, however, with the establishment of a nationwide system for the registration of private seals, which made the seals a requirement. Today, registered seals are used to validate legal contracts and other official documents.