Can anyone explain that the added ingredients do to the cake...
sour cream, what effect does this have on the texture of the cake?
what about pudding mix?
Usually extra ingredients are added to a cake mix to extend it so that there is enough batter to fill the deeper "decorator" cake pans (2 or 3") without using an additional mix.
Thread with all the extended/enhanced/doctored cake mix recipes:
(Also has a link to thread which covers just about anything that CAN be added to a cake mix.)
If you're interested in learning about the science of scratch baking and why certain ingredients are used in baking:
Everything you need to know to make, decorate and assemble tiered/stacked/layer cakes:
(Has popular CC recipes for crusting American buttercreams, fondant and WASC cake with flavor variations.)