Non-free software has no interest in its users being able to see how it works. They are supposed to just accept it, or to report problems and hope that the source code owners will choose to work on them.
Free software aims to work reliably just as much as non-free software does, but it should also empower its users by making its workings available. This is useful for many reasons, including education, auditing and enhancements, as well as for debugging problems.
The ideal free software system achieves this by making it easy for interested
users to see the source code for a feature that they are using, and to follow
through that source code step-by-step, as it runs. In Emacs, good examples of
this are the source code hyperlinks in the help system, and
Then, for bonus points and maximising the ability for the user to experiment
quickly with code changes, the system should allow parts of the source code to
be modified and reloaded into the running program, to take immediate effect.
Guile is designed for this kind of interactive programming, and this distinguishes it from many Scheme implementations that instead prioritise running a fixed Scheme program as fast as possible—because there are tradeoffs between performance and the ability to modify parts of an already running program. There are faster Schemes than Guile, but Guile is a GNU project and so prioritises the GNU vision of programming freedom and experimentation.