More than 60% of English words have silent letters.
Silent letters are the ghosts of pronunciations past.
They are borrowed words from other languages.
They change the pronunciation of another syllable e.g. 'fat'/'fate', 'hat'/'hate', 'don'/'done'.
when the printing press came to England, many of the printers were Flemish and German. They added in a little something extra to make the words look more like the way they'd pronounce them back home.
16th century academics messed around with spelling by wanting to make it more Latin and so added letters to words like debt, doubt and island.
That -gh- letter pattern is from the Anglo- Saxons - daughter, night, cough, dough, bright... the -gh- used to be -h- and pronounced like the Scottish loch, a hard sound - until the French invaded and added the g. Then the -gh- became silent or pronounced with a 'f' sound.
Knife, knock, know, gnat, gnaw are all Viking words which used to be pronounced but the letters are left in there to see the origin and history of the word (in Sweden they still say the silent letter in knife kneefe)