These are commercial genius. As they only just cover interest, you hardly ever pay anything off, leaving you near perpetually in debt, accruing more interest for years.
Borrow £3,000 at a typical 17.9% and make minimum repayments and it can take 27 years to clear at an interest cost of £4,000. Use the minimum repayment calculator at mse. me/minrepay to see how long yours will take.
Yet there’s a trick here too. Minimum repayment in month one is £70, so if you fix your repayments at this every month (rather than minimum repayments which reduce as you owe less)