What Is a “Portable” App, and Why Does It Matter?

One or twice a year I erase my operating system on C:\ partition and then rebuild it so that I have a clean OS that I know is reliable. To reinstall the OS I use an image file that I know is good and is a very minimalist system in terms of apps installed etc. This means that while I'm rebuilding the rest of the system I would normally not have access to Firefox and Thunderbird (for example). However, I have my portable apps on my E:\ partition and I can use them even when I am rebuilding the OS system on C:\ (so I still have access to the internet and email while I'm rebuilding the system on C:\). It's also good from the point of view that it means, even on a full-blown working system, I'm not installing tons of apps to the C:\ partition -- most of what I need is already available to me on the E:\ partition; I only install to C:\ what is essential to be installed there. I find this a great time-saver when rebuilding the system and, in any case, I really like portable apps anyway.