In general, imagine a problem space where you have millions (or much much more) of timeseries, each is potentially millions long (usually it's purely floats), and you want to perform time-series specific operations, like interpolate, extrapolate, moving avg, forecast, alert if ususual, plot, etc. Like, imagine AWS has 100s-1000s (or more) of metrics per "thing", and there's a very large number of things (EC2 instances, subnets, SageMaker instances, network switches, etc). Very specific data model, usually append-only, very specific read operations.