It primarily removed Punkbuster support (which was already obsolete) and allegedly fixed a few Remote Code Execution (RCE) exploits.
Once installed, it can automatically keep your client updated to the latest community version. How to Install the 1.8 (CoD4X) Patch cod4 18 patch
Since Activision's master servers are often down or broken, CoD4X provides its own master server list so you can actually find games. It primarily removed Punkbuster support (which was already