Cheok,
I had the same question, so I thought about the scenarios which might lead
me to need to restore from backup. I'm sure there are other scenarios, but
my list includes these:
1) Development / testing purposes moving data from one environment to
another
2) A failure in the GAE environmen
+1 to Jim's points.
We work quite hard to ensure the availability and persistence of your data.
Important data is duplicated and persisted in several ways, such that an
entire data-center failing need not be a major disruption - this is the
power of massively-deployed distributed computing and
