When and on what occasion are seals necessary?

There are two types of seals: registered seals and personal seals.
Registered seals must be registered at the ward, city, or village mayor's office where the bearer resides to identify that the seals actually belong to the person registered.
In most companies, businessmen stamp their seals on documents they've written to signify that they will be responsible for the contents, or to signify officially that they have taken a look at them.
Thus, seals are necessary in business interactions.
Seals are also used to express agreement in contracts, to give notices to government offices, to receive designated deliveries, etc.
These days, signatures are also acceptable for receiving deliveries.
For general use, personal seals are adequate, but resistered seals are required for important business documents, particularly financial transaction contracts.