It's safe to delete, but it will be created automatically again too. But it should only be created if you run a release build with the -multilog cmdline option, or run a debug build. Otherwise it's not used.
Currently it doesn't contain a lot of information which is of much help. The new FS2NetD (for 3.6.10) will make better use of it though, to help identify and diagnose problems. Other parts of the networking and multiplayer code will also end up being modified for 3.7 in order to make considerably better use of it as well.