I thought to learn in a previous discussion that use of 'la' is not optional and in this case should mean 'the (this particular piece of) meat?
Manzo is a bullock. Carne can be of any animal (including humans)
Because "e" ending words can be either M. or F. so in this case carne is F. so we use la for "the F.". For "e" ending nouns you just have to memorize the gender.