A bar code ensures that your book can be scanned at the point of sale in a bookshop and you will find it difficult to sell your book / have it accepted in shops without a bar code. The bar code contains the ISBN data.
There are specialist softwares that can be downloaded to generate a bar code but be wary of ‘free’ programmes that may not produce the quality required. There are also plug-ins for Adobe InDesign etc. that can generate bar codes. However Biddles can supply a bar code based on your ISBN when you ready for your book to be printed.