It runs against you as well

The same arithmetic applies to what you owe. A balance at twenty per cent a year, unpaid, grows exactly the way savings do — interest earning interest on itself. That is why credit card debt is different in kind from other debt rather than simply more expensive than it.

Which gives the ordinary rule of thumb its actual reason: clear high-interest debt before investing, because paying off twenty per cent is a guaranteed twenty per cent return, and nothing available to you offers that with certainty.