It can be very difficult to jump into the middle of someone else's code, especially if that code is poorly commented / annotated. Plus you can easily break things if you aren't aware of all the dependencies that may be linked to whatever you might be changing. I mostly worked in SQL and it was years ago, but sometimes I still say a silent prayer for whoever the poor SOB was who might have had to try and make sense of the hacky bullshit I put live on our production servers. Half the time I wasn't even sure why it worked, just that the output was correct.