When you install mods that use the official REDmod framework, the game compiles the scripts and stores the binary output in this cache folder. If your game crashes after installing multiple mods, deleting the contents of r6\cache\ (NOT the folder itself) forces the game to rebuild the cache from scratch, often solving conflicts.
The r6 folder is often the source of "corrupted or missing scripts" error messages:
Because the r6 folder deals heavily with compiled scripts, it is often ground zero for game startup crashes after a patch. Game Crashes on Startup