To prove the importance of secure ciphers, here is the tragic story of Mary Queen of Scots, who was foolish enough to use a weak cipher in the 16th century, long after codebreakers had mastered frequency analysis.
To cut a long story short, Mary wanted to assassinate Queen Elizabeth I, and began exchanging messages with her co-conspirators, in particular Anthony Babington. This was dubbed the Babington Plot. Their messages were so treacherous that they were enciphered, so that they could not be read if they fell into the wrong hands.
Here you can see a mock-up of the letters that were sent between Mary Queen of Scots and Anthony Babington:
The cipher that Mary used is shown below. It has a cipher alphabet, with substitutions for each letter from A to Z. The cipher also contains some code symbols for the most common words, and some more sophisticated symbols. This mixture of codes and ciphers is known as a nomenclator.
Mary's messages were captured by Elizabeth's spies and they were cracked by her chief codebreaker. Mary was immediately arrested, put on trial and the deciphered messages were used as evidence of her treachery. She was found guilty and was executed in 1587 ... all because her cipher was cracked.