MailArt was the only way for many people in communist-ruled countries to contact the outside world, especially during the Cold War. Irrespective of this, MailArt was nothing other than an exchange of art that took place by letter. So it made sense for creative people to decorate their own mail with their own stamps. Neither the test of the post nor the saving of postage stamps were in the foreground. The letter should arrive, after all, and artist's stamps in the sense of this MailArt were/are always affixed in addition to the normal postage. But you can't rule out the possibility that some people need the "kick".
My definition: "An artist's stamp in the sense of MailArt is an expression of an artist's opinion in a very small space. The techniques used for this range from rubber stamps (even multicoloured) to purely copied stamps to one-of-a-kind stamps produced partly or entirely by hand."
The spectrum of subjects covered ranges from personal expression of opinion, political, cultural in nature to the most diverse artistic directions one can imagine.
MailArt was, if you like, the forerunner of the internet at the time, but still lives on today, despite fast computers.