Desserts are usually too sweet. I want them to have enough sugar that they feel like a dessert, but it's common to have way more than that. Some of this is functional: in most cakes the sugar performs a specific role in the structure, where if you cut the sugar the texture will be much worse. This means that the cake layers will often be sweeter than I want for the average mouthful, and adding a layer of unsweetened whipped cream brings this down into the range that is ideal. It's good in helping hit a target level of sweetness without compromising texture.

(This is a flourless chocolate cake with precision fermented (vegan) egg.)

I also really like how the range of sugar contents across each bite adds interesting contrast!

Cream isn't the only place you can do this. I like pureed fruit, ideally raspberries, to separate cake layers. Same idea: bring it closer to balanced while increasing contrast.