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.
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: RimWorld.v1.4.3549.rar
Searching for "RimWorld.v1.4.3549.rar" typically points toward archived versions of the game found on file-sharing or pirate sites. If you are looking for a "good piece" regarding this specific version, here is the context on why it matters and what you should know. What is Version 1.4.3549? RimWorld has one of the most active modding