Like everything in life, if you have two of something the easier your life will be. Say you have two cars. You wake up and one does not start, well no biggie you have another car and take the other to work( The chances of the other one not starting are very slim). You will worry about the other one later. :). There is no difference in the computing world.
Say Firefox goes bad temporarily and you don’t want to re-boot and re-starting Firefox process is not working. Well no worries, just start up a copy of Opera, or Safari. I actually have all major browsers installed. Firefox is my main one, and occasionally it does go bad, thus I use the other ones. Of course, none of other have my bookmarks, or saved passwords so if you need to do other things more then just browsing you will be stuck.
But its a safe bet to have two of each thing installed. I pretty much have two of everything. I have open office installed and office 2007. And yes office 2007 has stopped working on a couple of times and when in a hurry and need to get something typed up fast open office write does the job excellently. I don’t stop my progress if a program acts up, I just another one.;)
Having two things installed is also a good way of diagnosing if something is not working. Say you are browsing and a page doesn’t work. You can’t tell if the web page server is down or if its your browser not working. Well just open up another browser and if you get the same error, well most then likely its the web page server that is not working… Of course this is assuming your internet connection is working( you can see other sites, besides this particular one).