When working in emacs, especially in large org-mode files and completion lists, garbage collection pauses cause the experience to feel sluggish. By deferring garbage collection until there is no user input (e.g. you stop typing or step away), the Garbage Collector Magic Hack (GCMH) keeps the Emacs feeling snappy.
The fact that you can control the garbage collector from within the running program using the garbage collector is a great examples of why emacs is the ultimate editor building material.