English words have many silent letters, such as the k- in knife and the -gh- in night. Old English was spelled the way it sounded. Then there were some pronunciation changes as the Britons learned to speak the language of the invading English. It changed again as the Danish and Norman invaders added to the language. When people wrote down words they often kept older spellings, even when pronunciation changed.
The word knife comes from the Old English cnif, which was borrowed from the Old Norse knifr. The word knot was from the Old English cnotta, originally the k/c would have been pronounced. The k-sound was eventually dropped by lazy English speakers. So the initial kn-sound got reduced to an n-sound.
The silent -b in climb, comb, lamb... was pronounced in Old English, climban, camb, lamb. However the silent -b in thumb and limb were not part of the history of English spelling (originally thuma and lim). The modern spelling may have been added by teachers so that the spelling matched other words ending in -mb.
The silent-gh got added by mistake to some words such as delight, originally from the French delite, so that it would be similar to the words night, right and light. Their spelling is related to its history, originally ninht, riht and liht, but the h-sound got dropped by lazy English speakers.
Many children learning English spelling dislike these silent letters, but they do sometimes have an accidental use. Sometimes the old spelling was kept because it helped keep two similar sounding words separate, for example: knot/not.
Sometimes English spellings were badly corrected by reforming teachers to new, but awkward spellings. These teachers tried to change English spelling by matching it to similar Latin words. They often made mistakes and created problems that were learned by others. Children still suffer from the mess these teachers created. I'll cover that fake Latin mess next week.