GTIN
A GTIN, or Global Trade Item Number, is the standard identifier behind a product’s barcode. It names the item itself rather than your listing of it, which is what lets two different stores be recognised as selling the same thing.
Without a shared identifier, matching has to be done on text, and text is unreliable in exactly the cases that matter. Two listings of the same item can differ in wording, capitalisation, pack size and colour name, and a matcher that gets it wrong either drops you from a comparison or puts you in the wrong one.
For products you make yourself there may be no GTIN, and an MPN with a consistent brand name does the same job. What does not work is leaving both fields empty and hoping the title carries it.
Why it matters to a store
Comparison is where an agent decides between you and someone else. An unidentifiable product is not compared badly. It is left out.