Silent Letters

Silent letters are letters that you can't hear when you say the word, but that are there when you write the word.

-Silent letters can distinguish between homophones, e.g. in/inn; be/bee; lent/leant. This is an aid to readers already familiar with both words.
-Silent letters may give an insight into the meaning or origin of a word, e.g. vineyard suggests vines more than the phonetic 'vinyard' would.
-Silent letters help to show long vowels e.g. rid/ride
-Silent letters help to show 'hard' consonants e.g. guest/gest
-They can help to connect different forms of the same word e.g. resign/resignation.

taken from:

http://languagelearningbase.com/88832/silent-letters-examples-and-spelling-rules