One of the main reasons for this is to buy Bitcoin anonymously. You can buy gift cards using cash and then use those cards to buy BTC without exposing your bank account or personal information to risk.
Read more ---> https://bitcointalk.org/index.php?topic=4541498.msg40867156#msg40867156

There are many ways to get some cuts, but trust remains a crucial factor.
Many sites impose restrictions to make sure you do not buy illegal cards ex Bitify.com (semi-Darknet bitcoin market.)