Suppose that your application is running on multiple machines. Some event occurs concurrently (at the same time) on two or more machines. If the machines time is not synchronous (machine 1 says 01.09.2013 08:00:00.000 and machine 2 says 01.09.2013 07:58:45.123). The events will not have the correct appearence in your log (or display, or whatever)!
If you ask a time server for the timestamp, then both events will have the same timestamp respectively all events (no matter on which machine) will have the timestamp from the same source! Timestamp could be important (event order, priority, and so on).