When something is sempiternal, it seems like it's been around forever, like the rise and fall of the tide on the beach, or your love of chocolate.
Sempiternal is useful for describing something endless, especially when you want to use an impressive word. Although it's often used the same way you'd use the word "eternal," in philosophy there is a distinction between those terms. "Eternal" implies something that is infinite outside the bounds of time, like God, while sempiternal is a more earthbound way to talk about forever. But you should feel free to tell your annoying little brother, "Your presence in my room feels sempiternal."