It’s easy to forget the steps you took to do something on your computer, especially several months later when you’re trying to upgrade. Sometimes when you try several different ways of solving a problem, it’s easy to forget which method was successful the next day!
It’s a very good idea to take notes about the software you’ve installed and configuration changes you’ve made. When editing configuration files, it’s also a very good idea to include comments in the file explaining the reason for the changes and the date they were made.
This has saved me so much grief. If ever I mess up a system so badly that I want to re-install or when I want to set up a new machine, having a clear set of notes makes it a breeze.