Cal here from Shopify, thanks for reaching out to us. This is a great question!
SKU's are useful for identifying products within your business. They entirely optional, but you can assign an SKU to any product you create on Shopify. If you venture to any product detail page, you will see the option to edit the SKU:
You can also use the bulk editor to achieve your SKU edits even faster:
You also get to decide the formatting for the SKU. We have some great tips on selecting your SKU format here. It's important to keep a format that is simple and understandable in your reports.
Let me know your thoughts!