Why dig in the dirt when you can carry a mushroom in your pocket!
I tried to be haughty about desserts in jars and how it's such an obnoxious fad like taxidermy in bars and guys who ride bikes and grow beards. But then I had an amazing goat cheese and berry mousse in a jar and changed my mind. Put it all in a jar! All of it! I take it back! Jars are charming and it could probably be scientifically proven that they make food taste 25% better.
This taps right into that toddler part of the brain where you want to pick up a whole pie and smash your face into it. Taking a big ol' spoon and jamming it into a jar of cake and frosting and having no regard for the mess you're inevitably going to make sounds so childishly satisfying. Where's my big spoon?