Crashed your computer’s Operating System, need to access data residing on the hard disk. Broken hard disk, need to access internet to send some important e-mail. Need to debug your hardware, but don’t want to play with your current operating system.
Live CD is the answer to these problems, live CD contains an operating system with which you can boot your computer and use all the available computer resources. If you are worried that it can alter your current operating system , no it can not. Once you have the operating system running from the live CD, try exploring different options.
There are many utility specific live CDs which are designed to perform just a few functions. It could be for gaming, multimedia ,education or forensic.
There is list of such live CDs, However my favorite list is here