Good question! We use a lot of ingredients that don't penetrate the skin well, like hydrolyzed oat protein, carionic polymers, and oils, to name a few, and we know they offer great benefits.
I think we need to talk about what it means for an ingredient to penetrate our skin. Most ingredients will penetrate a layer or two of the stratum corneum, or the upper layer of our skin. Ingredients and products will mix with the natural moisturizing factor or the stratum corneum lipids to enhance moisturizing and hydration, assist with exfoliation, or create an occlusive layer. We can add ingredients to our products to help increase the possibility of penetration, called penetration enhancers. (Never ever do a search for that on Google!!!)
To answer your question, there are loads of benefits to using Vitamin C in a product. Very few of our ingredients offer any penetration beyond the stratum corneum, and that's okay!