First, because you get access to the biggest retail marketplace on the planet. Having an extra store in your hands, you don't have to worry about where to sell your products.
Second, selling on Amazon is a hassle-free (compared to traditional business channels) business. You just log into the store, enter your business credentials, list your product, and start making money. Within a few clicks.
Third, you can own multiple Amazon stores multiplying your profits and protecting yourself from putting all eggs into one basket.
That's why the demand for reliable high-velocity business stores that can withstand massive inflow of orders has quadrupled during the COVID-19 pandemic.