The explanation is better if it allows you to explain a large number of similar words arising from a common source by a systematic process.
If you have to make up a new just-so story for every pair of words, of course you're not gaining much, but if the same story works for many words at the same time, positing a common origin isn't too far-fetched.
If you have to make up a new just-so story for every pair of words, of course you're not gaining much, but if the same story works for many words at the same time, positing a common origin isn't too far-fetched.