A lot of symbologies support some kind of modulo check digit. It's mostly there to detect erasure and substitution errors, because those are relatively common errors in decode. A modulo sum is better than nothing, considering that each additional digit increases the length of the barcode. If you're length-constrained, then adding more check-data is a difficult trade-off between stronger protection and smaller module size, meaning that you could add so much data that the barcode becomes too dense to print or read.