I usually only and always choose Update existing notes when first field matches. Leave it like that for all decks, if you like. That is what I do.
Then every import I do just updates the contents — e.g. if I am changing the card’s presentation, or correcting or tuning the information on a card.
The above implies that I always create the first column as a unique identifier.
Some people, prefer not to create an additional field to use as a unique identifier. They then prefer to allow several different cards with the same first field, as they may be e.g. synonyms. For example: you might have create a card whose Front field is ‘white’ and whose Back field is ‘weiß (adj.)’. Later, you wish to add another card, when you realise that the English word may have another meaning, so you add ‘white’ (same Front field) and Back field ‘Eiweiß (n. masc.)’. Thus you want both cards.
The left-most column is taken as the unique identifier, in the sense that if Anki finds ‘white’, it will want to overwrite the contents. Thus people without an additional unique identifier (e.g. 23546), will wish to choose Import even if existing note has same first field.