Today, the biggest value you'll get from States is the ability to segment the major parts of your workflow logic. For very simple workflows it's easy to stuff everything into the starting State. As your logic becomes more complex, it becomes increasingly difficult to deal with visually. At some point you'll end up zooming out and everything is too small to read, or you end up panning around the flow so much that you lose location context. If you break up your logic in to different States you'll have a nice way to dealing with manageable parts of the workflow on the screen, and in your head too.
There's not much value to inbound email flows with reporting on these States, and as such there isn't reporting available yet, because those flows are constrained to be short-lived. In the future there might be other types of flows that could last days, weeks, months, or more - and those flows would have more of a benefit to knowing what State their in at the moment. An inbound email flow might end up lasting a few seconds, or tens of seconds, so there's not much value in knowing which State it's in because that information is likely stale by the time you know it. But a flow of some future type that lasts three weeks would benefit more from knowing that a particular instance spent XXXX hours in State-1 and then YYYYY hours in State-2, etc.