Why is the SKU field required, and do downloadable and affiliate products need one too?

SKU is required on Simple and Variable products. Wcart uses it to track stock, including stock per warehouse and store location. The form shows "SKU is required." if it is empty, and the field suggests one you can use.

Downloadable and Affiliate products have no stock, so their form has no SKU field. Wcart creates a unique SKU for them automatically when you save.

On a Variable product, the main field is SKU (parent). Each variant can have its own code in the SKU column. Leave a variant's SKU blank and Wcart creates one from the product and option names.