Those directories are where the cached copies of your forum files are. I am really not sure how/why multiple directories get made, it's usually in a forum that's been transferred, upgraded (someone's upped a new vars_config.cgi file), etc. They also hold your search indexes, so deleting extra ones could save some major space
Saving additional space :
- make sure images are optimized for size
- if your forum started out as a v5 series ubb (since upgraded to v6 series), you can now safely go through and delete the Forum#/HTML folders from each forum as they are no longer needed/used - that can make a huge difference if you were a v5 forum for a while.
- that's about it
