what is cake filling for? What else would you want to put on the cake before you add the fondant or b.c.
filling and frosting should enhance the taste of the cake, it's nice to mix up the textures too, creamy or silky filling with spongey cake.
I personally find that even moist cake goes down easier with creamy buttery frosting goodness.
When I put BC on a cake I crumb coat it, refrigerate and then top coat it.
when I put fondant on a cake I crumb coat it then fondant it. You could use an apricot glaze or brush the cake with syrup instead, you just want something for the fondant to stick to.
I hope that answered your questions, I am not a pastry chef so there might be a different reason for filling cakes.
I also like to use filings to accent or contrast the flavor or texture of the cake. For example, I have a custard-y pudding I use with chocolate cake, as it is not very sweet and compensates for the sweetness of the cake itself, and the outer frosting.
If nothing else, filling is used to glue the two, three layers together!