In Asia, painters stamped with their own seal as a statement of completion (as a signature). Artists might used their name, the season, or places for their seals. They use the stamps to balance the ink paintings.
Some historical paintings had printed with many government seals because there was approval for viewing. That explained some of the museum pieces had lots of different sizes or shapes and might not find another one alike. Nowadays, some of the Chinese masters still fellow the tradition to add on their own seal to show their appreciation toward their student’s works.