Shakespeare called z an “unnecessary” letter, but the letter c is probably a better candidate for the title.
In modern English, c is a substitute letter, a stand-in symbol for two English sounds that have distinctive letters to represent them. These sounds are /k/ and /s/, as in cat and cent.
C wasn’t always a mere substitute for the letters k and s. In Old English, c was the only symbol for the sound /k/. A note in the OED explains what happened:
When the Roman alphabet was introduced into Britain, C had only the sound /k/ ; and this value of the letter has been retained by all the insular Celts: in Welsh, Irish, Gaelic, C, c, is still only = /k/ . The Old English or ‘Anglo-Saxon’ writing was learned from the Celts, apparently of Ireland; hence C, c, in Old English, was also originally = /k/ : the words kin, break, broken, thick, seek, were in Old English written cyn, brecan, brocen, þicc, séoc.
In OE, as in Modern English, the letter s represented the sounds /s/ and /z/.
Before the end of the OE period, c became palatalized before e and i. Meanwhile, changes were going on in French spelling and pronunciation. Among other things, French adopted the letter k to represent the sound /k/ in some words. The Norman Conquest of England in 1066 spelled more than political disaster. The Norman scribes who ousted their English counterparts were accustomed to Latin and French spelling conventions. When the Normans invaded England, English spelling went from consistent to what it is today.
After the Conquest, c kept the /k/ sound in some English words like candle, cliff, corn, and crop, but in others, the sound /k/ was spelled with the new letter k, as in king, break, and seek.
Rule: In modern English, when the letter c occurs before the letters i, e, or y, it represents its “soft” sound: /s/.
Here are some examples:
C followed by e
accept (The first c in accept stands for the sound /k/; the second c stands for /s/.)
C followed by i
C followed by y
If you find exceptions to the rule, please share them.
Note: This stated rule applies to the single letter c, not to letter combinations like ch, tch, or cious.