It's cheap insurance for you, your bow, your hunt, your tournament, etc.
I've heard of people going for years and years with the same string...and I've had people contact me because their string broke at the worst possible time and they were in a pickle. It's not cheap to over-night a string to a motel room.
Obviously when you see wear, it's time to change. If I shoot a particular bow much, the string gets changed at least once a year if not sooner. One problem is, as noted, you can't see wear under the serving.
High draw weights and/or low strand counts should be changed even more frequently.