It's simply a unique gift that boosts affection by +1% regardless of the girl's favorite gift (even cake).

Capsella's Homemade Cake

Some uses I could recommend:
1) Give it to a girl that has an odd-numbered affection %, to bring it back to an even number.
2) If you have plenty of it, use it to finish daily gifting quests if you want to reserve the regular gifts for future use.