Why?
Using Linux is a choice.
Just as there are hundreds of Linux distributions, there will be just as many reasons to use it (or not use it).
For example:-
Do you want a computer that starts up (boots) in 10-15 seconds?
Try Xpud or Moblin. (for netooks)
Do you want a system small enough to carry around in your wallet on a credit card CD?
Try Slitaz or Puppy
Do you want to install on something other than an X86 PC?
Try Debian.
Do you want to run a system that is immune to Windows security exploits?
Use any Linux. [or an Apple ;) ]
Can you not afford to buy 50 Microsoft Licences for the computers you have just had donated?
You can install Linux free on as many computers as you want.
Do you want to replace your old MS servers, but can't afford the new licenses?
Linux excels, (dominates even) in the server arena. It is free, or you can pay for support if required.
The list and the choices go on.
Choices from the O'Reilly online catalogue:-
http://oreilly.com/catalog/debian/chapter/book/ch01_03.html
Top 5 reasons on softpedia:-
http://news.softpedia.com/news/Choose-Linux-Click-here-to-find-out-why-43186.shtml
Ten things Techrepublic think are important:-
http://blogs.techrepublic.com.com/10things/?p=334
From Linux-watch, 5 reasons [NOT] to use Linux ;)
www.linux-watch.com/news/NS8124627492.html
Here are a few choices I make.
(1)
For financial transactions, I turn my computer off, then restart it with a linux 'live CD' in the drive. I can now access my account fairly safe in the knowledge that there is no trojan/virus/keylogger harvesting my account details.
(2)
Unlimited choice at no financial cost. Or to put it another way, I can play with as many different versions of Linux as I choose without spending any money.
This is great if you are interested in operating systems, but probably more of a problem if you are just trying to figure out what to use.
(3)
I do not use Linux (or windows) exclusively.
Computers and 'gadgets' in general are tools to get a job done, if they do the job, the OS (operating system) may not need to be considered.
I don't suppose many people consider what OS their mobile phone uses. Or if you have a broadband router, it could well be running the Linux OS, but that may not be of any interest or matter to you as long as it carries on working.
Finally, if you are completely new to computers, and you go on a course at the Library, college, or community centre, chances are that you will be taught using the Microsoft Windows OS unless you go out of your way to find Linux training for whatever reason.