Probably necessary to denote which side of the family the aunt is from.
Not probably. It's essential. I'm not sure if you are Cantonese.
If not, and for the non-cantonese here:
Goo je = younger sister on the dad side
Goo mah = older sister on the dad side
Baak leung = older brother's wife on the dad side
Sum sum = younger brother's wife on the dad side
Ah yee = younger sister on mum side
Yee man = older sister on mum side
Kau mo = I'm having difficulty with this one. It's like a family aunty. Perhaps someone can help.
And finally, just aunty = this usually reserve for your mum's friend or total strange!
That's us Chinese for you. Lol