A common seal or rubber stamp is basically used if you own a company out of Singapore. This can be a private limited company or a limited company. You'll need the common seal so that your company can issues shares to the shareholders or deeds. In Singapore, the common seal is also known as a company seal or a pocket seal. This stamp is used in most common law jurisdictions for share certificated and common deeds.
Apart from this, the common seal can be used in schools or universities that issue certificates for attending a class or completing a degree. But in Singapore, this is a necessity and you'll need the common seal if you own a business there.