It isn't really redundant.

Y'all is a missing distinct 2nd personal plural that English lacks but other base languages in romance and germanic language families it descends from have. Like "ustedes" or "vosotros" in Spanish, or "sie" or "ihr" in German, depending on formality requirements based on who is being addressed. Or yinz for yinzers speaking Pittsburgh-ese.

It resolves the ambiguity in "you" reused of the singular and plural forms.

So "I need all y'all to understand this" is equivalent to "I need all of [group of people people being addressed] to understand this" as opposed to "I need some of y'all to understand this". The "all" in "y'all" a quantifier to "you", the first "all" in "all 'y'all" is a distinct quantifier to the group represented by "y'all".

The ungrammatical part is that it should be "all of y'all" but such is idiom.