I did that as a kid on occasion and have done it as an adult to relive my childhood.

Don't knock it until you try it. In the right proportions, the sugar of the jelly mixed with the fat of the peanut butter, creates PB&J flavored frosting.

For example eating a stick of butter is gross or eating a cup of sugar is too much. But mix them together and you get frosting which when spread on cake is amazing.