The timing belt is made of rubber and fabric, and it doesn't last forever. When it breaks, the valves slam into the pistons, everything gets tangled up, and the engine dies dead. It's called an 'interference engine,' and that's what we're blessed with. So the maintenance schedule calls for the timing belt to be replaced at intervals of 105k miles so that this won't happen.
Unfortunately, the timing belt also turns the water pump, and if its bearings give out and it seizes, this breaks the timing belt. I know. It happened to me on a previous (non-Subaru) car. The silver lining is that this got me to buy a Forester 6 months earlier than planned.
So, the general recommendation is to religiously replace the belt at the scheduled intervals, and while you have the engine opened up that far to also replace the water pump and a few other associated components. Search around here and you can find more discussion of what ought to be done as part of that job.
2003 XS Premium MT >200k miles! ● 2005 Impreza 2.5 RS MT ● Robin-Subaru Powered Generator