Actually, this practice among the most conservative of catholics goes way back. But the practice of inflicting a bit of pain isn't limited to catholicism;I think Buddhists have their version of it, too. My understanding is that the pain goes beyond symbolism, it somehow sharpens the feeling of spirituality on the sufferer's part. It makes sense when one thinks about endorphin release. Hey, I don't understand what would compel someone to run 26 miles but there must be something that makes it enjoyable.
I'm Catholic and forever I have read many bizarre and gruesome stories in the Bible and books about the lives of Saints. This is a very old practice, and widely used from way back. It is my understanding from some of the stories, self-flaggelation is a form of humble submission and relating to the suffering of Jesus on the cross. It is used to atone for sins, and also to achieve a higher state of spiritual consciousness. In any case, it is seen as a sign of holiness. But, it is an old old practice, beginning with the earliest Christian martyrs who professed their faith and sought joy in their torture and death.
If you read the life stories of St. Augustine, St Teresa of Avila, St. Francis of Assisi, Mother Teresa and so many many saints - you will find these saints to have practiced hundreds of examples of mortificaton of the flesh, from mild (wearing burlap clothing) to severe - sleeping on a bed of nails with a rock pillow every night.
This is the stuff that is incredibly bizarre to read about but also very real. No one taught me about such practices when I grew up in the Catholic church - we were taught about penances that were acceptable for the common lay folks such as fasting and denying yourself of your favorite foods or whatever. So, I didn't know about these things until I started reading about the lives of Saints - what an eye opener!
Additionally, there are many writings on moritification of the flesh by popes throughout the years, even fairly recently.
and yes skunky points out that these things are not exactly confined to Catholic spiritual practices. there are many other groups practicing self-denial and pain. but I think Catholic mortification is unique in its origins, practices and beliefs - based on biblical and historical teachings.
If by poverty your definition means being in charge of a city/state, ultimate ruler of millions of followers, living in a palace surrounded by guards, wearing designer robes worth thousands, jewelry, private jets, and a vehicle named after you. Then yes he took a vow of poverty.

you are a total nutcase. a vehicle named after you! designer robes!!!

yes, those robes are very expensive as they are handmade using the finest materials.