Since Steam automatically updates games, a .rar backup is the only way to ensure a specific state of a colony is preserved forever. A Word of Caution If you are downloading this file from an unofficial source:
.rar files from third-party sites are common vectors for malware. Always run a scan before extracting.
Users typically look for specific version archives for two reasons:
The core of the Biotech DLC, allowing for hereditary traits and xenohumans. Why use a .rar file of an old version?
RimWorld has one of the most active modding communities in gaming. When a major update (like v1.5) drops, it often breaks older mods. Players download specific archives to keep a "frozen" version of the game where their 300+ mod collection still works perfectly.