Your stock is linked to the products you sell, so every sale and every restock updates the same number.
Because your stock and your sales use the same product list, your stock count always matches what you've actually sold.
Each product has its own stock count. It goes up when you restock and down when you record a sale.
Yes. Products running low are flagged for you, so you don't have to check every item yourself.
Yes. Instead of only adding stock, you can set the exact number, which is handy after you've counted everything.