The default VM2 cart variant custom fields don't have an option for field validation which would require the user to make a selection from the dropdown first before they can add it to the cart.
Let's say you sell t-shirts, and then you have a Cart Variant custom field called "Size", with dropdown options of -Please Select-, S, M, L, XL, XXL. It would then require the user to select a size before they can add the product to the cart. This should help any accidental product option purchases.
This is a standard feature of many other eCommerce systems and I feel VirtueMart should contain this functionality by default.
Looks like this feature was added to Break Design's "Custom Fields for All" plugin recently:
http://breakdesigns.net/extensions/customfields-for-all
Don't think I'll be using the native VM cart variant custom fields anymore.