Barcodes are not mandatory but strongly recommended if you intend to sell to retailers or online marketplaces.
Some of the companies that use and require GS1 barcodes include Amazon, Tesco, Sainsbury's, John Lewis, WHSmith, Unilever, Ocado, Nestle, Google, Fruugo, La Redoute, New Egg, NHS, Walmart, Macy’s.