This folder is a catch-all for profile manipulating and processing operations. The Redux store, reducers, and selectors are not the proper place to do complicated work manipulating data structures, especially something as complicated as a performance profile. The functions in this folder operate completely independently from Redux and React components. Typically they are wired into various selectors and different places within the overall app to perform the various pieces of work, or to compute derivative data structures.
For instance the marker-timing.js file takes the list of markers from the processed profile, and computes timing information for laying them out easily in the Marker Chart. This is a derived view on the data that is then hooked up into a selector, so that the computation gets memoized and cached whenever it is used throughout the application. The marker timing functions can therefore be pure functions that return a new derived data structure. This architecture makes for functions that are easy to test, document, and that work consistently and quickly in the application.
The profile processing functions live here, and are documented in Upgrading Profiles in the /docs-developer folder.