This feature is needed to analyse how cell's value is produced by formula.
divides formula into sub-expressions and evaluates each
one. You see value of each expression and it's parts recursively.
For example, we have the following formula A4=A3+A2+A1+1. On the first step
we'll see following expressions:
After expanding A3+A2+A1 node:
You can expand all nodes recursively, to see evaluation of other
cells(arguments) and expressions.