There are many reasons why you may use applications on a flash drive: for fun, or because your pc is crashing or because you want to work on somebody else’s computer.
There are a huge number of portable programs available, many of them free. There are portable browsers, graphic editors, media players, and programming tools such as utilities. I keep a flash drive with assorted portable diagnostic tools that come in handy when someone asks me to help them out with their computer. I have chosen portableapps.com that has Firefox as a default browser and Clamwin as antivirus.