"The wicked is a ransom for the righteous" (21:18). What does that mean?

The whole saying is, "The wicked is a ransom for the righteous, the treacherous for the upright" (21:18) (tap ransom). It doesn't say who pays the ransom, to whom, or when. Another saying has some of the same Hebrew words: "A righteous person is delivered out of trouble, and the wicked takes his place" (Proverbs 11:8). Both have righteous and wicked, and in 11:8 takes his place includes tachat, the word the WEB gives as for before the upright in 21:18. Proverbs has ransom twice more: "He won’t regard any ransom" (Proverbs 6:35), and "The ransom of a man’s life is his riches" (Proverbs 13:8). The chapter doesn't explain 21:18 further, and these notes don't add to it or apply it to anyone.