"All y'all is used in the Southern United States when a speaker wishes to include everyone being addressed. Y'all may refer to an indefinite set of members of a group, but all y'all definitively includes everyone in the group." https://en.wiktionary.org/wiki/all_y'all
Having fairly recently moved to the South, I hear y'all all the time. I thought it was cute until I read a linguist's take on the term and now I think it's wonderful and am trying to adopt it into my lexicon. She explained that it's nothing more than a contraction for "you all," is gender neutral, is respectful to young and old alike, and is friendly and inclusive. While many (including some Southerners) view the term as boorish, it sure beats the New England "youse."
A guy I know who grew up in CT and moved to WV now says "all" instead of "oil." If you've been to Boston, you will be amazed how they have removed the "r" was every word and still manage to communicate (Pak your ca in Havad Yad). John Kennedy took language lessons to try to enunciate the r sound, with limited success.