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To get better lifetime out of the cells, you don't fully charge them. I bet when bad weather is occurring soon, it charges to maximum storage capacity, not to max cycle level.

This lets you have best of both worlds, with a minor tradeoff in unpredictable outages you have slightly reduced runtimes.

At peak generating times, it may be valuable to sell your stored energy to the grid. If bad weather is coming, you might not want to do that.

Sometimes you discharge. You may decide to not discharge ahead of a storm. If it were just backup, it would be easy, but sometimes you want to consume your storage to save money, or sell back to grid, if you have time of use pricing, or get paid for grid service.