There are hundreds of rules to help figure out how to spell and read words, but they do not work all the time. There are more exceptions than rules. :)
Silent letters show up because English has a lot of influences. Many words in the English language are derived from foreign words. English takes words and spellings from many different languages. A lot of times, when there is a silent H at the begging of a word, it comes from a rule in French language pronunciation. That’s why hour sounds like our. Silent Ks come from Old English, which was spoken in England before 1100 CE. They actually used to be pronounced, but over hundreds of years, the sound was dropped, leaving us with the words knife and knight, where we don’t pronounce the first letters.
Since accent and pronunciation differ, letters may be silent for some speakers but not others.