Why do old documents have a postage stamp stuck on them?

I have come across some old solicitors and Revenue documents dating back to early 1900's and most of them have a postage stamp stuck on them near the signature.
Why were these put there?

In the past clerks in the courts affixed stamps to legal documents when they were filed with the court. They are tax stamps to show that the proper filing fees and taxes have been paid for that transaction.

It is the tradition of offices that in every document is stamped datewise and sign. If any higher authority want to investigate the matter, the documents will detect the person who was on duty. For new date stamp you can take our help at http://www.laserwritepro.com