Purpose of the binary-all architecture folder folder on debian repositories?

From the Debian Policy Manual:

Depending on context and the control file used, the Architecture field can include the following sets of values:

• A unique single word identifying a Debian machine architecture as described in Architecture specification strings, Section 11.1.

• An architecture wildcard identifying a set of Debian machine architectures, see Architecture wildcards, Section 11.1.1. any matches all Debian machine architectures and is the most frequently used.

• all, which indicates an architecture-independent package.

• source, which indicates a source package.

And from the Packaging Best Practices:

6.7.5. Architecture-independent data

It is not uncommon to have a large amount of architecture-independent data packaged with a program. For example, audio files, a collection of icons, wallpaper patterns, or other graphic files. If the size of this data is negligible compared to the size of the rest of the package, it's probably best to keep it all in a single package.

However, if the size of the data is considerable, consider splitting it out into a separate, architecture-independent package (_all.deb). By doing this, you avoid needless duplication of the same data into eleven or more .debs, one per each architecture. While this adds some extra overhead into the Packages files, it saves a lot of disk space on Debian mirrors. Separating out architecture-independent data also reduces processing time of lintian (see Section A.2, “Package lint tools”) when run over the entire Debian archive.

So, typically the binary-all packages are data files, or executables which are architecture independent (like scripts). By keeping these separate, the load on the repositories is reduced.