Depends who you ask. It is ostensibly to take away or reduce sexual pleasure ... it is supposed this will cause women to be more faithful to their husbands.
The logic of this escapes me. If you cannot have sexual pleasure at all, a new lover wouldn’t help matters.
Please, the people who generally practice this act are Sololies and some souhtern Egyptians — and animists (Pagans)in sub-saharan Africa. This is not something Arabic or Persian Muslims practice or Asians.