What are stamps used for?

• "Stamps in Indochina". Last modified June 1, 2016 (21 May 2022)."data-thumb="https://augustusinsaigon.uni-trier.de/files/thumbnails/0e0673421cbf0ca603d02fde5c6498e4.jpg" data-download-url="https://augustusinsaigon.uni-trier.de/files/original/0e0673421cbf0ca603d02fde5c6498e4.jpeg" class="media resource">
  
• "Postage Stamp". Last modified December 3, 2020 (10 May 2022)."data-thumb="https://augustusinsaigon.uni-trier.de/files/thumbnails/d8ae86686969a8356c2a3810f5db409a.jpg" data-download-url="https://augustusinsaigon.uni-trier.de/files/original/d8ae86686969a8356c2a3810f5db409a.jpeg" class="media resource">
  
• "Bao Dai on stamp". Last modified April 10, 2022 (21 May 2022)."data-thumb="https://augustusinsaigon.uni-trier.de/files/thumbnails/6ed1f6386e5b2321704e4fcdd0b4048d.jpg" data-download-url="https://augustusinsaigon.uni-trier.de/files/original/6ed1f6386e5b2321704e4fcdd0b4048d.jpeg" class="media resource">