"the Great English Vowel Shift, [which] took place in the early 1400s. Before the shift, a word like loud would have been pronounced 'lood'; name as 'nahm'; leaf as 'layf'; mice as mees'. ...The renewed interest in classical languages and cultures, which formed part of the ethos of the Renaissance, had introduced a new perspective into spelling: etymology. Etymology is the study of the history of words, and there was a widespread view that words should show their history in the way they were spelled.... that it would help people if they could 'see' the original Latin in a Latin-derived English word.
So someone added a b to the word typically spelled det, dett, or dette in Middle English, because the source in Latin was debitum, and it became debt, and caught on. Similarly, an o was added to peple, because it came from populum: we find both poeple and people, before the latter became the norm. An s was added to ile and iland, because of Latin insula, so we now have island. There are many more such cases."
I find it interesting that the biggest spelling challenges in our language -- the "inconsistencies"-- the silent letters -- the irregularities --are there because someone studying etymology, at some time in the far past, thought they were helping future English users see the connections between related words.