Changing the timing belt is just GOOD maintenance for a car over 100K...
When the engine runs, the timing belt maintains the "timed" relationship between the crankshaft and the camshaft. As these turn, all the internal components of your engine are moving in time with each other (via the timing belt). If you break the timing belt, all those internals go out of sync and start banging against each other, pretty much destroying everything...
For the health and longevity of your engine, it should be replaced. It's a helluva lot cheaper than a new engine
Chris