Many misunderstand the classical lesson as "substitution ciphers are trivially broken by symbol frequency analysis", which isn't the point. The point is to ill

Many misunderstand the classical lesson as "substitution ciphers are trivially broken by symbol frequency analysis", which isn't the point. The point is to illustrate a property of a cipher that leaks information, in this case the symbol frequencies because the cipher preserves them. This is information that we don't normally consider valuable when working with plaintexts, but for crypto it's enormously valuable (i.e. it leaks a lot of information).