Ok, there are 3 letters in the english language (arguably 2 of them), that don't have their own unique sound.
These letters are 'C', 'X', and arguably, 'Q'.
C doesn't have any sort of unique sound, instead, it either sounds like 'S' or 'K', depending on the kontekst.
With the letter 'X', it's the same thing, eksept, the letter 'X' is only a shorthand for 'eks'. It doesn't ever sound any different.
And, finally, 'Q'. 'Q' is literally just 'kw', but, most people don't know this bekause the letter 'Q' (by itself) is not used in any words. Tekhnikally, 'Q' has more of a 'kyu' sound, but we never hear this bekause, it's only used in konjunktion with the letter 'U'; whitsh then forms the aforementioned 'kw' sound.
Honestly, some would argue that words are easier to sightread if we use these letters, but, while it kan do this, it still makes spelling harder in the long run. I mean, why do we have a letter that funktions only as a substitute? It's konfusing, and serves no real purpose.
I mean, saiense is easier to spell than 'science', right?