You sell a wide range of products. Websites are capable of showcasing far more products than a bricks-and-mortar shop, and if you have a lot of different items on offer, an ecommerce site will probably be the single most efficient way for you to sell.
You wish to market to a wider audience. If you’re only selling from a physical location at present, you’re only reaching a tiny percentage of the market (i.e. those within travelling distance of your shop). Even if you have multiple locations, that’s still nothing compared to the reach of an ecommerce website – imagine what could happen if you made your products available to the entire country, or even the entire world!
You already have a non-ecommerce website that gets a lot of traffic. Either way you need both.
You are selling in a specific niche market. I.E. Book