Solar generation is less predictable, and the grid needs an energy source that is capable of quickly adjusting to the current fluctuation in demand of consumers.
That's where the batteries come in. To continue the software metaphors, batteries provide caching, and the cache is tunable for cost and performance. This wasn't viable a decade ago, but with the advances in solar/wind cost/performance, and the advances in battery cost/performance, it's really becoming a powerful option.
