The inventor of Morse code, Samuel Morse, needed to know the frequency of the letters of the alphabet in English so that he could give the simplest codes to the most frequently used letters. He did it simply by counting the number of letters in sets of printers' type. However, for word games, it is often the frequency of letters in English vocabulary, regardless of word frequency, which is of more interest.
https://www3.nd.edu/~busiforc/handouts/cryptography/letterfrequencies.html