The snapshot will capture exactly what you brewed that day, and it has several uses. I'll make a snapshot of every brew day to preserve the exact recipe that's tied to the brew session and tasting notes. If I use a substitute hop, I alter the additions in the snapshot to match the target IBU, leaving the base intact for next time. That snapshot is saved so I can go back and see where I've made that substitution and how it turned out.
It's also useful for when you need to change the base recipe somewhere down the road. I brewed several verisons of a hefe, compared them side by side, and copied the features I liked best from each snapshot recipe back into the base.