SILENT SOUNDS in English words... what's the reason behind it?

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My 9 year old niece and I are reading some books and we encountered some very simple words like "should" and "would", which I explained to her that we should read it with the silent "L". And she asked me, "how come?". I was at a loss for words for this. Do you have any idea why? What's the sense of putting silent sounds in English words? Why can't it be read plainly and simply as how it spelled? Now as I continue to read, words like psychology (silent p), subtle (silent b)... etc. keep on popping up. -lol- Either I forgot the answer to my own question or my mind doesn't seem to work right now... -lol- Your words of enlightenment is greatly appreciated.