Strictly speaking, it is "Calling (visual) subroutines from within a function node in the same execution context"
It permits someone who is writing code in a function and needs to call-out (to grab a value/format a thing) to take advantage of re-usable code (be that something in a library of utility functions in a function node OR a little utility flow that uses other core/contrib nodes OR ...).
Man, it is difficult to easily explain or portray the possibilities that it opens up, but hopefully it doesnt take too long to realise the doors opened by this).
The graphic and demo flow in this comment might provide some additional context. There was also a rather long and detailed discussion in the discourse thread that discussed various options and approaches to the whole reusable question. The first post is worth a read to understand the rationale. I also know it will not be what suits everyone nor provide solution to every scenario, but it was the best of a bunch of options in terms of what it brings vs effort etc.
