Johnbee58
...I don't even want to think of how long it would take me now if I ever had to reformat/rebuild or replace my HDD or perhaps even the whole computer with a new one. It would take WEEKS, for sure!...
One important set of files to keep a backup copy of is the sample content, so that the sample content can be easily reinstalled whenever necessary by copying over a folder rather than running an installer. This can save a lot of hassle if the sample content has been added to since its initial installation.
For example I have had Dimension Pro and Rapture since 2007 and have added various expansion packs and also created some additional content of my own. To install all this again from scratch would be a horrendous task. Fortunately I have never need to do this as I have always kept a backup copy each time I added an update. Note that, for these two softsynths, it is necessary to backup "Program" files as well as multisamples.