Anurag’s example is a good one…
Another common example of a use-case is to use the factor of an instancing spline to control the lengths:
This is not useful way of thinking about fields as it doesn’t have general application (even though you can do it as demonstrated by KDLynch and StrayBillie). Anurag’s example and mine show more useful approaches where you care less about the spline’s index and more about its context in how/where it is generated.
No, this is a red herring - You use Accumulate fields for other things (like stacking boxes).
Good luck!