The point is you don’t always have choice , some supermarkets only have self serve or they only have 1 manned checkout open . Personally I don’t like or use self serve I’d rather wait in a queue to be served by a checkout operator.
I don’t look at size tags anymore , if it fits it fits I wear size 16 - 20 depending on the garment & fit . Size is just a number if it worries you cut the size tag out .