For me event-sourcing is a historical layer below your existing database that allows you to replay the sequence of events that got your database to its current state.
If I had to businessify it, maybe it would be 'Wolkenkit is a framework that allows you to save and replay your entire business's history so you never miss a key insight again.'
From a technical perspective, engineers can focus on the product and organically build and destroy the data structures around a changing business instead of trying to find the perfect data-structures that will inevitably be incorrect as the business grows and evolves. You get regression tests for free, and you get backups for free which may be nice-to-haves when the business is small, but absolute requirements once you have real customers paying real money. Not sure how I would squeeze that in an elevator pitch though.
Greg Young's presentation on event-sourcing is pretty great, you may find some inspiration there: