Why would anyone want to pay off their debts in a way other than “highest interest” first?
There are a number of bona fide reasons to consider here. If there is a cost to discharging a security packet, or a mortgage, it may not be convenient if we are advanced in the repayment schedule. Early exit fees may apply, or the interest may be "pre-determined". As a rule of thumb, when we are talking about rates above 10% p.a. then arrangements should be short (bridging finance - keep it short and charge 'em heaps), and for personal arrangements, small.