Right, anytime the developers choose to change any file (whether it be adding new content, fixing a problem in the code, deleting content, etc), it can be considered a "patch". The newly changed and updated files are what you are downloading in the patcher when you first load it.
Code doesn't just "go bad" after a while. Programming is a very complex beast and oftentimes involve a ridiculous amount of logic (loops, loops within loops, conditionals, etc). As with most things that involve such amounts of logic, it's easy to accidently make mistakes and it's damn near impossible to find small mistakes when you have tens of thousands of lines of code to look through. Say, to figure out why the certain monster is healing itself too much, doing too much damage or running away when you originally intended to have it stay and fight until it dies.
That is why as games get more complex and large, they oftentimes require patching (fixing those small or large mistakes in the coding) to correct some of the smaller mistakes that the programmers may have missed. And as we've seen with some games, it can take years of patching and searching through code to finally get most of them. And adding in new features can cause new problems in the code as well.
