Many of these files contain "cracks" that antivirus software flags as malicious. While some are legitimate community patches, others are used as wrappers for malware.
Before extracting any .rar file from an unknown source, upload it to a malware scanner.
Bypass restrictive NAT types that prevent players from hosting lobbies.
Ensure players on different versions (or those using different storefronts) can join the same sessions.
Developers frequently release major updates that render old "fixes" obsolete.