As an UV protector, you can find it as octylmethoxycinnamate or Tinosorb OMC used to up to 10%. It is an oil soluble sunscreen that works to protect skin from UVB rays. It's generally found in anhydrous products like lip sticks or sports sunscreens. The cinnamic acid we find in shea butter, for instance, can behave as a mild sunscreen, but I wouldn't trust it for that purpose without testing!
Cinnamic acid can behave like an AHA (alpha-hydroxy acid), which means it can penetrate the skin and help with cell regeneration under the skin (more on AHAs in the future). Some call it anti-aging as it can help to reduce the look of fine lines and wrinkles through this mechanism, but it can lead to slightly thinned skin and a slight increase in transepidermal water loss with use over time. But the wonderful and moisturizing fatty acids we find in something like shea butter are more than enough to compensate for these concerns!
Cinnamic acid and cinnamic aldehyde can cause contact urticaria, which is a skin irritation syndrome which results in an immediate type reaction of the skin or mucous membrane. It can be as subtle as itching and redness or as severe as weals or flares on the skin. As little as 0.01% cinnamic aldehyde can produce this result in sensitive people. (This may be one of the reasons someone is sensitive to shea butter.)