Why would people use one of them rather than increase the fee? Is it just for "forgot to do it, I'll pay extra now" situations, or is there a reason to do it anyway?
In some cases, increasing the fee once the transaction has been broadcast is not possible (original transaction doesn't use replace-by-fee OR child-pay-for-parent is not possible)
As someone replied I researched a similar service years ago to get a friend's transaction unstuck. Usually just a credit card payment and the tx id and they mine it.