All the processing power in the world wouldn't matter much if you couldn't get output from a computer. You're looking at output right now on your screen. You told the computer you wanted to view this page and the software and microprocessor inside it responded by putting the page on your monitor.
Other kinds of output include sound from your computer's speakers and documents printed by your printer. Output can also include things like MP3 files. They allow you to download music from the Internet onto an MP3 player you can take with you anywhere.