Cake is good" needs no qualifiers, no asides. Cake needs no promotion.
Cake needs no PR. Good goods, like cake, should be treasured, appreciated, & kept moist as long as possible.
Frosting, icing, & fondant form a hydrophobic barrier around cakes, preventing moisture loss.
Once you cut the cake, you remove its protective barrier & expose it to moisture-leaching air, resulting in dry cake. You can prevent this several methods.
America's Test Kitchen suggests storing the cake with a peeled apple beneath a cloche.
Apple acts as a cake humidifier, preventing it from drying out. Even under a cloche, fruit attracts flies, so I prefer apples.
TikTok user austrianwithwuff suggests using parchment paper instead of fruit. Parchment paper (or wax paper, or plastic wrap) functions as a hydrophobic barrier.