Add customized options and extra services (free or paid) to your products and offer them to your users.
If you sell your products online, you most likely need advanced selection options. For example, if you sell rings or jewelry, you might need to let your customers add a custom text to be engraved in a piece of jewelry or choose carats for a gemstone, the size, the color for gold.
If you sell T-shirts, cups, and other custom items, you certainly need a field to let your customers upload files, like their photos, during the checkout process. Or if you run an e-commerce site for tech products, you might want to offer warranty and assistance services for an extra cost or allow users to select a large number of options like RAM, processor, screen size, weight, etc.