Letter frequency analysis is used in a substitution cipher to help decrypt the encrypted text. In the English language, the letter 'E' accounts for about 13% of all letter. This is an average of course, as shorter texts or phrases may not reflect this, but the more encrypted text available to analyze, the more likely the averages will appear as expected.
Therefore, letter frequency analysis can help determine the real letters that have been obscured or replaced within the cipher.
For more information: Letter Frequency @ Wikipedia