As a letter "Й" is used to make sound like Y in Yes, but its accurate to say that combination "Ye" from Yes sounds like russian "E". There are several russian letters that sounds like "Й" + another letter, for example E = Й + Э, Ю = Й + У, Ё = Й + О. Thus, the main purpose of "Й" is the formation of new letters. :) Sorry for english