As computers take on more and more automated tasks, they seem to spend more and more time fully powered on. In many cases personal computers are left running all the day just to execute some tasks that the computer may do during non-working hours.
For example, a disk utility may be set to defrag the hard disk during the middle of the night, or as we move into the era of the digital home, a computer may be left running so that the PVR software can record the user's favorite tv show.
Certainly there are tasks that require the computer to be always on, but for other tasks there is often a better method.