Screensaver = Fanbreaker + Noisemaker + Batteryuser
From the Brian who is not yet disillusioned with his iBook to the one who was actually pretty lucky considering how hard he is on things...
Once upon a time, screensavers actually saved the screen. Back in the days of CRTs, images could get burned into monitors, and so it was useful for the computer to show flying toasters as the classic After Dark screensave did. I used to have a Star Trek: The Next Generation screensaver for my Mac IIsi. If I stepped away from my machine, it would show samples of the green and orange displays that surrounded the bridge and sickbay of the Enterprise.
Today in the world of LCD displays, this is not a problem. Yet people still like their computers amusing them when they feel ignored. So manufacturers still supply screensavers, even on computers like my iBook whose screen does not need that kind of saving.
What needs saving on a laptop is the battery. Unlike my old ThinkPad, the iBook is smart enough to leave its fan off until the temperature of the CPU really becomes almost hot enough to damage it. Then the fan kicks in until it's cool.
This means that if you set the screensaver to something that uses a lot of processing power that will heat up the CPU, the fan can kick in.
In fairness to Apple, I do have custom power usage settings, not the defaults, and my iBook was plugged in when it activated both the screensaver and fan to get my attention after I ignored it for too long. When I'm using the battery, it shuts the display down before the screensaver can kick in.
It's fascinating to me that something that was once useful with a bit of whimsy has become 100% whimsy and then antithetical to its motivation: using software to protect the hardware.
In the really dark old days of DOS, when the operating system was not expected to provide this feature, and even before After Dark provided it system-wide as a utility, I wrote a screensaver feature for some data collection software I wrote that could be used for up to eight hours at a time. Since it was running in a medical research lab, I thought it would be amusing if it displayed graphs of bogus information, with the axes labelled with meaningless buzzwords. The bogus graphs were sometimes linear, sometimes logarithmic, and sometimes scatterplots. I don't know if it ever fooled any visitors into thinking that we really were measuring antimatter containment.