Many reasons. You might not know how to get a high quality rip(Itunes default is only okay), you might not be able to locate the CD when you're putting together an Itunes playlist, the CD might be damaged or lost. If it was an Itunes download, you might have lost it when your hard drive crashed.
The people and bands who are it purely for the music will continue making music and playing music regardless if they make a profit.
There will always be music, whether or not people make money. But will they pay for orchestras, the best production, etc.? I mean, if you are okay with metal CDs sounding like they did in 1996 when there were no sales, that's fine, but if you want to hear bands like Rhapsody, Avantasia, and Nightwish give you the whole epic experience, they need to have a big budget.
Same goes for movies and games. If you like independent films and Space Invaders, a non-profitable industry will not affect you. If you like big summer blockbusters and awesomely produced games, then people need to step up and buy them.
Mind you, I'm actually pro-file sharing, but we need to use it as a tool to help us find stuff to buy, not use it as a replacement for buying.