Cackle is a tool to analyse the transitive dependencies of your crate to see what kinds of APIs each crate uses.
The idea is look for crates that are using APIs that you don't think they should be using. For example a crate that from its description should just be doing some data processing, but is actually using network APIs.
Is this a problem today? I’ll scan through the code of less popular crates, looking for reasonable TCP std library usage and such, just to be sure, but I haven’t found unusual networking crate usage (yet).
That said, the repository may not actually contain the code compiled in the crate. I’ve found situations where the source code is impossible to find.