There are many reasons to run programs on a PC without installing them.  Probably the greatest reason is when your computer crashes due to malware or corrupted programs, and you need to diagnose the problem and attempt to fix it.  If you have problems with your operating system, you may not want to make things worse by attempting to install new utilities on top of the problem.  A better way is to have a USB flash drive handy with a bunch of programs already installed that can run on any Windows-based computer.
Another reason may be that you’re traveling without a computer, but want to run your own apps on a hotel, school or business center’s computer, and keep your own bookmarks, passwords and personal data safe from the public environment.  Running a portable web browser, bookmark manager, and password manager can all be done from your flash drive, leaving no traces on the computer that hosts the apps from your flash drive when you remove it.
The two suites of programs I use and recommend are:
Portable Apps have been around for a number of years.  LiberKey is the ‘new kid on the block,’ but has incorporated quite a few enhancements over Portable Apps including automatic synchronization of your Flash Drive’s programs with your account on their web site, and automatic download and updating of the programs you already have.
Each suite comes with a self installer and an initial set of programs. Portable Apps has a little more than 200 apps listed on their web site along with an app that can make other applications portable.  LiberKey has over 300 apps listed on their web site.  Both suites include utilities (including file management, diagnostics & anti-virus/anti-malware,) audio, video, games and productivity applications.
One area I’m interested in checking out in the near future is seeing whether either or both of these suites will run in the BartPE portable Windows environment that will boot directly from a flash drive. Check back when I post an update of my investigations.