It depends on how massive your business is and where you’re selling.
If you have only a few product types, store only a few dozens, and sell them at a small brick-and-mortar store, you may not need an intricate SKU system until you scale up.
Amazon FBA requires FNSKUs. No matter how many product types you have, when you’re using Amazon FBA, technically, you’re already using a SKU system.
If your inventory has about 100 different products or it can already fill an entire room, then it’s time to consider building a SKU system. It doesn’t matter if you’re selling only at a physical store or using an online platform.
However, this is only a rough guide. Keep in mind that spending too much time doing inventory is already a reason for creating a SKU system.