Hi,
Thanks for your feedback.
Well, there are many cases where you don't want the quantity of the main product to be set to the sum of the quantities of the variants.
For example, suppose you are selling books. Each product is a book, and each one has two variants: ebook and physical book. The physical book variant might need to have a limited stock while the ebook and the main product can be set to unlimited.
Also, even without the plugin, if you set the main product quantity manually first, it will be updated when variants of the product are sold.
But yes, in many cases where you need to have the stock in both the variants and in the main product, this plugin is really great to have.