The circuit length is used to calculate voltage drop. Calculating voltage drop in Revit is still not always possible due to the way it automatically assigns wire size. I wrote about that here: Revit Idea: Change how wire is auto-sized or allow override for engineering judgement.
Turns out there is a bug in how Revit calculates circuit length...
when shared nested families are used. Whats more, this existed in previous versions, it is just easier to find now that the path can be made visible.
The image below shows a panel, a device and the circuit path in a 3D view. I also turned on the Project Base Point (found under the Site category). You can see the circuit path goes to the project origin rather than the farthest device.
Nested families with electrical connectors are not to common, so this will not likely be a big deal for most. And not at all if you are using something like SKM PowerTools to design voltage drop and arc flash outside of Revit (meaning, you don't use the circuit length data reported by Revit). However, this technique (i.e. shared nested families with electrical connectors) can be used for things like rooftop units with two load requirements (voltage/phase).
The image below shows a correct circuit path in the same model. Thus, the issue only occurs for individual circuits using shared nested families.
I happen to know Autodesk is aware of this issue... and I am sure it will be resolved in the near future.
BTW, I also wrote about the new Edit Circuit Path feature in my What's New in Revit 2018? article on AECbytes.com.