The latest patch contains all the changes from the previous patches, addresses a few small game bugs and importantly adds improved Windows Vista compatibility with Doom 3. Doom 3 has also received a couple of patches, bringing it up to Version 1.3.1. The game's programmers often resolve certain issues, improve particular aspects of gameplay, or add new features through such patches.
It is traditional for any modern PC game nowadays - no matter how well it's coded - to receive at least one patch during the course of its life.
